I don't think that if a 51% attack happens, then the coins in addition to Bitcoin will live, immediately a panic will start in the market and everyone will sell Bitcoin, because no mistakes should occur, and canceling transactions is quite the opposite of what Bitcoin was created for.
Of course, maybe in the future there will be a new coin that will be protected from such manipulations, but it will take at least 5 years to restore trust for people, I think so
If you think governments care about Bitcoin at all, to the extent that they're willing to spend billions on trying to harm the current Bitcoin, then you are wrong. You cannot prevent 51% attack but the opportunity costs that arises from this is immense. If it ever happens, then we roll back to before the attack and change the algorithm. By doing so, the government has effectively wasted billions of dollars for nothing.
51% attack is not a Bitcoin killer because the game theory behind it hasn't failed. The sheer energy required to execute the attack is equivalent to certain smaller countries. The land area required to house the ASICs and the manpower required to set it up would be fairly big as well. Without understanding the difficulty of executing an attack, and the benefits of doing so from the government's PoW, then you won't understand why this isn't a problem.
Agree with some of what you said, but governments absolutely do care about Bitcoin. When you have major enterprises like VISA, Amazon, Twitter, banks, etc. starting to use it and it poses a threat to the central banks fiat currency, they are starting to take notice. That's why you hear all this noise about stablecoins, because governments see this as a way to destabilize crypto and a direct threat to CBDC - they're missing out.
A few billion to destabilize Bitcoin would absolutely happen, if governments thought it would work. While you couldn't practically change the algorithm, people would switch to a different crypto before it happened, making the effort pointless. Bitcoin is one serpent head among the medusa's head of snakes. They can try to cut off one head, but PoS or something else would just take over.
If every major country banned/bought out cryptos, they could theoretically discourage it, but I think they see the futility in that and are going to try to tax it somehow. Too many elites making money off of it and manipulating the masses into buying/selling with influencers like Musk/Bezos/Dorsey flipping the switch and making things go up/down.