Both of the possibilities below don't seem to be things that it would be that easy to realise even once we're in some (near-ish) future. I hope this isn't begging the question, it isn't intended to be. I've put credences and I'm glad you asked for them, but they are very uncertain.
One possibility is that we were just wrong about the whole long-termism thing. Given how much disagreement in philosophy there seems to be about basically everything, it seems prudent to give this idea non-trivial credence, even if you find arguments for long-termism very convincing. I'd maybe give a 10% probability to long-termism just being wrong.
More significant seems to be the chance that long-termism was right but that trying to directly intervene in the long-term future by taking actions that were only expected to have consequences in the long term was a bad strategy, and instead we should have been (approximate credence):
- Investing money to be spent in the future (10%)
- Investing in the future by growing the EA community (25%)
- Doing the most good posible in the short term for the developing world/animals, as this turns out to positively shape the future more than actually trying to. (20%)