You list a bunch of claims that mostly seem over-optimistic and that show no sign that you have thought critically about whether they are actually true. 

Being able to communicate with everyone means we can travel and study and work anywhere.

No, immigration law is a stronger barrier to travel and work permits than languages are. 

Since education is the key for getting out of the malicious poverty cycle, this will bring fundamental change to a major portion of the world’s population.

Education is a lot about signaling. Imaging that Western education itself without the signaling value is able to change the lives in developing countries doesn't have a good base. 

When there's a language that everyone is proficient in, contracts will be much clearer, resulting in more trust in oversea partners and confidence in foreign trading. 

Do you have any evidence that language of making contracts is any barrier to business? When it comes for example to China, the way contracts are enforced seems to be much more central than the wording of the contracts when it comes to trust.

But the world's economy will actually benefit the most from the leaps-and-bounds advances in science and technology. The IAL fosters much better collaboration between scientists from different nations, now that they can understand their colleagues' research published in a foreign magazine.

English works for that purpose and most fields get centered in a way that makes the important communication English.

More generally, if you would introduce a new IAL, you wouldn't start with it being spoken by everyone. To get it adopted you need to have it be useful if not everyone speaks it. 

This is a beautiful set of ideas which bear further discussion.

ShiraDest

Gain = (Benefits − Costs) ∗ Probability

Would be more like gain = benefits*probability of those benefits - costs*probability of those costs, especially if there are failure modes that exist. I'd also try to avoid framing it as "benefits are almost unlimited while costs are finite;" while an IAL is great, the benefits of an IAL are just as finite as the costs are.

That being said, I think that if you can make an IAL that is exceptionally good on many dimensions and get enough interest/funding behind it, it would be an extremely worthwhile project.

English is the current "language Z" currently, but I agree that the language Z could be better. Are you planning on designing an IAL? I only do loglangs, so I can't help (or help to a meaningful degree), but I'm interested in reading your plans if you do have plans.

I'll write more about the failure of Esperanto and how I think we can do better.

In my opinion, the difficult part of things like this is coordination. There are already hundreds, maybe thousands, of constructed languages. Most of them probably have less than dozen actual speakers, i.e. people who can fluently keep a general conversation. And yet, many people decide that the best way to solve this situation is to create yet another constructed language. (Definitely not to support the one that already has a few orders of magnitude more speakers than the remaining constructed languages combined.)

So it seems to me that you either assume that coordination is easy in general, or that making the right design choices in your language will have a very powerful effect on all potential speakers. The former seems disproved by evidence; the latter kinda by outside view... consider the hundreds of authors of the other constructed languages -- they probably also believe that the impressive features of their languages will irresistibly draw humanity to their project -- what exactly is it that makes all of them wrong and you alone right?

You've probably noticed that your post has negative points. That's because you're clearly looking for reasons why an IAL would be great, rather than searching for the truth whatever it may be. There's a sequences post that explains this distinction called "The Bottom Line" [LW · GW]. Julia Galef also wrote a whole book about it called "The Scout Mindset" that I'm halfway through, and is really good.

That said, having an excellent IAL would obviously be a tremendous boon to the world. Mostly for the reasons you gave, scaled down by a factor of 100. And Scott Alexander and I think also Yudkowsky have written about the benefits of speaking a language that made it easier to express crisply defined thoughts and harder to express misleading ones---which is an entirely separate benefit from "everyone speaks it".

One of the biggest pieces of advice I would give my past self is "start small". I find it really easy to dream of "awesome enormous thing", and then spend a year building 1% of "awesome enormous thing" perfectly, before realizing I should have done it differently. When building something big, you need lots of early feedback about whether your plans are right. You don't get this feedback from having 1% of a thing built perfectly. You get much more feedback from having 100% of a thing built really haphazardly.

Putting that all together, my advice to you---if you would accept advice from a stranger on the internet---is:

• Stop thinking about all the ways in which an IAL would be great. It would be great enough that if it was your life's product, you would have made an enormous impact on the world. Honestly beyond that it doesn't matter much and you seem to be getting a little giddy.
• Start small. Go learn Toki Pona if you haven't; you can learn the full language and start speaking to strangers on Discord in a few weeks. Make a little conlang; see if you think there's something in that seed. See if you enjoy it; if you don't you're unlikely to accomplish a more ambitious language project anyways. Build up from there.

As a note on this:

I reason with myself that, since the Benefits to the world are almost uncountable while the Costs are finite, even if the Probability of success is small, the Gain for humanity is significant. Something along this line: (9e15 − 7e13) * 1% = 89.3e12 ($)

This is not valid upon reflection because an identical argument holds for any other action, say, everyone buying a tomato. If actions have their effects multiplied by a large number, then we should look for relatively good actions. If the chance of buying a tomato having big effects is small but the gain is really large, then we should look for more effective paths to big gains.

I was delighted to run across this post; I joined lesswrong so that I could reply to it!

There have been some pivotal changes since the push was made to adopt Esperanto as an IAL; perhaps the idea should be explored again.

One of these changes is AI.  It should soon be possible to make an "AI friend" -- basically a glorified tablet, perhaps with a simple arm - that could serve as a baby monitor and speak to the infant in the IAL.  The child would learn to speak the language effortlessly if it is well-designed and completely regular.  The AI friend could also teach the child to read the language, as well as some arithmetic, colors, animals, measurements, etc.

This would greatly reduce the enormous costs of teaching children an IAL in school, while doing a better job of it.  One of the problems with Esperanto back in the day was that even if we had succeeded in teaching every kid in the world Esperanto, it would have been spoken a bit differently in each country, and there would have been further drift from there.  But every child learning an IAL at a young age from an AI friend has the same teacher, eliminating regional differences and drift.

Such a dramatic reduction in cost changes the calculus.  Another change to the calculus is provided by climate change and the low birth rates in many countries, both of which will stimulate massive immigration.  Immigration is vastly more difficult if the immigrants do not speak the host country's language; the costs are enormous.  For one thing, immigrants often are less economically productive because of the language barrier, so they are poorer, and they pay less taxes to the host country.  (As an example, I know a surgical nurse who is immigrating to the US but will likely clean toilets with her mother because she is having a hard time learning English.) When we are talking about tens or hundreds of millions of immigrants, the costs of such reduced economic productivity are enormous.

Moreover, immigrants who don't speak the host language tend to live in enclaves rather than assimilating into the larger society.  There are many such Spanish-speaking enclaves in the US.  This too has consequences.

I'd love to chat more about this; I have written privately about the costs of teaching the world's children hundreds of different natural languages and then dealing with the difficulties that causes, but am looking to engage with others with the same desire to see the world learn a common language.