But yes -- even if they succeeded, as many people have already considered (I remember Antonoupoulos describing the aftermath of a potential attack very well many years ago) -- a reorganisation would undo the attack. Remember, success isn't one single 51% attack to create an altcoin. Success means also convincing everyone else the altcoin is the one everyone would follow. Colussion also involves conversion.
2) Specific measure against a "rival blockchain attack": Identify a third blockchain directly competing with the attack blockchain (in the case of Ethereum being the attack chain, for example Solana, Cardano or Avalanche). So those invested both in the attacked and the attacker blockchain can dump their stakes on the attacker blockchain buying the third chain's coins, reducing the attacker blockchain's value and increasing the third blockchain's value. The third blockchain's whales will very likely not participate in the attack, because they will benefit much more if the attack blockchain crashes and they can get the market share.
Now that I've thought about it, I think it might be dangerous for Bitcoin investors to switch to a third PoS coin (fully or partially) as a solution. The danger is that if they are not extremely coordinated, the first investors to buy the third altcoin might get them for cheap whereas the last ones to move will have to pay a lot more BTC for them. This could cause a great shift in the wealth between the individual Bitcoin investors.
It would probably be much better to them create a brand new PoS coin and simply copy all the wallets and balances from Bitcoin to that one. Note that this is similar to making a PoS hard fork of Bitcoin in practice. Afterward the investors can trade the new coin in order to establish its value compared to Bitcoin (the PoW version) via the normal market forces.
What do you think of that?