This topic has been discussed so rigorously and the conclusion is always that no reasonable state or adversary would spend so much resources on trying to stop something so insignificant. Unless you're looking for any sorts of profits, 51% won't work. You are bound to spend a few billions in both the hardware, facilities, electricity and various other miscellaneous costs for such an insignificant attack. The money could be better used in other areas. You COULD diminish the confidence that people has in Bitcoin but another crypto would simply rise again and the cycle continues. Not to mention that ASICs are not that easy to obtain or operate in bulk.
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