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.