Ethereum already has quantum resistant algorithms ready to go for each part of the system but they perform worse so it doesn't make sense to start using them until quantum computers are more real. There is no long term threat from quantum computing to Ethereum or other systems like it.
Money without an army is just paper. Or, in this case, imaginary numbers.
There will be no one to cry to when the machines start cracking all the hashes and wipe away the entire ecosystem.
I'd wager the NSA already has this capability, but they won't blow it on something so stupid.