>despite those millions of civilizations having fundamentally different mental architectures, ecologies, circumstances, time scales, and difficulties in spreading out into space is really a tough sell
Absolutely, which is why I said "an answer" as opposed to "the answer".
Personally, I think "it's just inevitable that all civilizations die from ecological collapse" takes some of the emotional frisson out of the doom and gloom as well; if it's just that inevitable, you can't really blame anyone, can you? If it's inevitable, it isn't really "fossil fuel's" fault (after all it is likely that many civilizations never even had fossil fuels, or lacked an oxygenated environment or similar that makes combusting them so easy), or careless use of farm land, or anything else... it's just the inevitable workings of the impersonal universe.
As with many other putative answers to the Fermi paradox, I don't think this one is powerful enough to explain it.