What Should I Do?

By "success" I meant successfully doing useful research/getting money to charity/spreading rationality. Which I think is closer to "value creation" or "positive externalities" than what you seem to be calling "success," namely personal welfare.

Actually, I didn't mean only personal welfare. What I wrote certainly applies to career success that brings money and power, but also to all other kinds of accomplishments that are commonly seen as worthy and admirable.

In fact, often it's even more difficult to judge the latter accurately. People are used to arguing whether someone who profits from success has deserved it, and they're normally willing to listen to someone who argues either way in some particular case. However, when it comes to accomplishments that are seen as selfless idealism, it's much more difficult to criticize those without being perceived as weird or malicious. This despite the fact that these can be about signaling games, rent-seeking, and unaccounted externalities just as much as any for-profit endeavor, no matter how admirable and high-status they are commonly perceived.

what if the funding for research in a field is limited, and there is someone who is better at doing research than you are, but worse at doing job interviews, so by deciding to go into that field, you take their place?

Even in the worst case, I suspect I am much better at doing research than the marginal person who would lose their job. Maybe there are some weird other issues with imperfect candidate selection, but I don't see why those would have any effect on average.

In reality, I expect the situation to be much better, for two reasons. For one, even holding the total amount of research funding fixed, how do you think that the allocation between various fields or subfields is done? I believe the number of competent researchers/grant-writers working in a field effects the fraction of funding it captures. For two, most people in most fields pursue programs which are optimized for maintaining status, not for doing good. You shouldn't expect the amount of socially optimal research to be conserved unless funding agencies are very good. In practice they actually seem to be astoundingly bad (at least for this way of evaluating good/bad). I guess these points are the same, but at different levels: working on the best programs within the best subfields of the best fields is important, and I don't think the average person you statistically expect to replace is doing any of these.

if there's a profitable niche, it will be filled by someone sooner or later

I believe this for a sufficiently generous notion of "later," and for enterprises which create marketable value proportionate with the good they do. But I care about shorter timescales than niches generically get filled on, and my notion of value is quite different than most peoples. Evidence for the first point comes from the amount of money that goes to successful entrepreneurs. Evidence for the second comes from looking around at the world, or reasonable expectations of marketplaces.

In both cases, I think others' remarks are essentially correct: markets work to distribute labor evenly in some sense, but my motives are different from my competitors' and so I suspect I will displace people who are doing substantially different things.

I found out that if I work in some field I'll get bored after a time-frame of months to years, and this was not something I expected when I was in college.

I guess I don't know if this will be a problem. I have definitely gotten bored of particular problems, but at MIT at least there are a pretty wide variety of problems being worked on and it seems pretty easy to move around. Maybe it will become harder to move around (I imagine I can avoid this) or a higher level of meta-boredom will set in which would require a more disruptive change.

Right now this is not high on my list of reasons not to go into academia, though it would be good to know more before spending years getting a PhD.

He didn't say he'd enjoy doing anything, he said there was a fairly wide range of things he enjoys.

That seems fair, but given your experience I hopefully don't need to rely on this form of blessing :)

Imagine a counterfactual version of your past self which did have the willpower to focus on one project at a time (but the willingness not to if required); could you give him advice which could counteract this disadvantage in disguise? Or do you think it is a matter of luck that this turned out to be a good thing?