The thing about 51% attacks is they're hard to pull off in secret. And once they happen, who's going to accept the coin anymore? Plenty of potential for sheer destruction, but it seems pretty counter-productive to value.
I bring this up because people are always asking what platforms are allowing me to short cryptocurrencies, which seems to miss that it's enough to just have a debt denominated in what you want to bet against.
With BTC's block reward continually being reduced, TX fees will have to increase in order to avoid reaching the point where large miners could become tempted to attack the network.
At some point, someone doing AI might amass enough GPUs to do a 51% attack on Bitcoin. You're right that it destroys confidence in the coin, so if you short Bitcoin futures before the attack, you might make money.
This is electrically impossible for Bitcoin specifically, modern ASICs exceed 3 orders of magnitude more hashes/Joule and hashrate/chip than a RTX5090 and cost $2-40 retail per chip.