Interesting thought. I think you have a point about coevolution, but I don't think it explains away everything in the birds vs. mammals case. How much are birds really competing with mammals vs. other birds/other animals? Mammals compete with lots of animals, why did only birds get smarter? I tend to think intra-niche/genus competition would generate most of the pressure for higher intelligence, and for whatever reason that competition doesn't seem to lead to huge intelligence gains in most species.

(Re: octopus, cephalopods do have interactions with marine mammals. But also, their intelligence is seemingly different from mammals/birds - strong motor intelligence, but they're not really very social or cooperative. Hard to compare but I'd put them in a lower tier than the top birds/mammals for the parts of intelligence relevant to the Fermi Paradox.)

In terms of the K-T event, I think it could plausibly qualify as a filter, but asteroid impacts of that size are common enough it can't be the Great Filter on its own - it doesn't seem the specific details of the impact (location/timing) are rare enough for that.

I agree it's likely the Great Filter is behind us. And I think you're technically right, most filters are behind us, and many are far in the past, so the "average expected date of the Great Filter" shifts backward. But, quoting my other comment [LW(p) · GW(p)]:

Every other possible filter would gain equally, unless you think this implies that maybe we should discount other evolutionary steps more as well. But either way, that’s still bad on net because we lose probability mass on steps behind us.

So even though the "expected date" shifts backward, the odds for "behind us or ahead of us" shifts toward "ahead of us". 

Let me put it this way: let's say we have 10 possible filters behind us, and 2 ahead of us. We've "lost" one filter behind us due to new information. So, 9 filters behind us gain a little probability mass, 1 filter behind us loses most probability mass, and 2 ahead of us gain a little probability mass. This does increase the odds that the filter is far behind us, since "animal with tool-use intelligence" is a relatively recent filter. But, because "animal with tool-use intelligence" was already behind us and a small amount of that "behind us" probability mass has now shifted to filters ahead of us, the ratio between all past filters and all future filters has adjusted slightly toward future filters.

Yes. Every other possible filter would gain equally, unless you think this implies that maybe we should discount other evolutionary steps more as well. But either way, that’s still bad on net because we lose probability mass on steps behind us.

Two objections:

1. Granting that the decision theory that would result from reasoning based on the Fermi Paradox alone is irrational, we'd still want an answer to the question^[1] of why we don't see aliens. If we live in a universe with causes, there ought to be some reason, and I'd like to know the answer.
2. "why aren't we born in a civilization which 'sees' an old alien civilization" is not indistinguishable from "why aren't we born in an old [alien] civilization ourselves?" Especially assuming FTL travel limitations hold, as we generally expect, it would be pretty reasonable to expect to see evidence of interstellar civilizations expanding as we looked at galaxies hundreds of millions or billions of lightyears away—some kind of obviously unnatural behavior, such as infrared radiation from Dyson swarms replacing normal starlight in some sector of a galaxy.^[2] There should be many more civilizations we can see than civilizations we can contact. 

1. ^{^}

   I've seen it argued that the "Fermi Paradox" ought to be called simply the "Fermi Question" instead for reasons like this, and also that Fermi himself seems to have meant it as an honest question, not a paradox. However, it's better known as the Paradox, and Fermi Question unfortunately collides with Fermi estimation.

2. ^{^}

   It is technically possible that all interstellar civilizations don't do anything visible to us—the Dark Forest theory is one variant of this—but that would contradict the "old civilization would contact and absorb ours" part of your reasoning.