(1) Capability is not the same as action. Every police officer in my city COULD murder me with their department issued gun at pretty much any time, but they haven’t. There are multiple reasons why, not the least of which is that _actions have consequences_. Worrying about that scenario is futile.
(2) The major cryptocurrencies aren’t as vulnerable to a malicious majority as you seem to think. All of the BTC ATMs, PoS providers, crypto exchanges, etc have strong incentive to ban malicious peers and they can do this soon after they identify the threat. The malicious majority would not be sufficient - they would also have to continually mine their own blocks faster than the rest of the network does.
(3) There would be a forked blockchain but only naive nodes which trust by default would continue with the illegitimate fork. Of the nodes who actually USE the cryptocurrency don’t agree with the malicious majority, it will be difficult to get the coins / tokens out of exchanges.
(4) The duration of any stolen nodes is the duration of the attack. Once the AI GPUs stop the attack and return to responding to LLM prompts, the legitimate blockchain returns to being the longest one, so all of the network returns to trusting the legitimate blockchain fork.
(5) The BTC network is controlled by a protocol agreed to by consensus. If the illegitimate blockchain fork stays longer than the legitimate one, the participants in the market can agree on a protocol change which hardcodes the illegitimate blockchain out of the picture (this happened with the ETH DAO in the early days after a successful double-spend attack).