Honestly we are not even talking about the real threat, which is the quantum computing and that's getting closer and closer, which would be fast enough to crack any of this encryptions. 51% attack is not a worry because not only miners do not have that much power right now, but also the fact that people can get together to prevent that against it as well is a big reason why it can't. We have seen ETC get that before, and look how they recovered.

Because people could just handle that when they get together, and that would be what BTC would do and in a big situation as well, multiple times bigger than ETC. So this is not an issue, there are plenty of ways proven to be working as a defensive mechanism towards anything like this ever happening.