My brain is not big enough to process this alone...

It is, you just won't put in the work. Nobody puts in the work.

Poof you have a quantum computer. You set it to mine bitcoin. Lookitthat it's hella faster than anything else. It earns its first bitcoin reward. Then its next. Then its next. Holy fuck we go from one new block every ten minutes to one every nine, every eight, every seven... what the fuck is happening? Before long Goobstercom is mining blocks every three minutes and nothing else can compete. It takes a couple feverish days of freaking out but 52 hours later, BTC forks into BTC and BTC-g, BTC-g being the chain that requires a six-month-old ASIC MAC address in order to access the blockchain. Things are controversial but 89% of miners follow BTC-g, while 9% stick with BTC because they are obstinate maximalists, 1% stick with BTC because they believe in the future and 1% stick with BTC because they went out and bought quantum computers and are trying to get in on the game. Meanwhile everyone who had BTC before the split has BTC and BTC-g and everyone is pissed off because they now need to get on the exchange and trade out their BTC for BTC-g and they also hate the new logo because it looks like it was commissioned on Fiverr six hours before the split (it was).

A consensus protocol is never going to give you stability. That's not the point. None of this matters, by the way, with a switch to proof-of-stake - the hashing ceases to matter, the power consumption goes away and the network speeds up tremendously.

But everyone will take one idea they don't understand - quantum computing - and cross it with another idea they don't understand - blockchain - and presume that because they didn't think of it, nobody thought of it and therefore we're all doomed. It's the tedious lovechild of Von Daniken Syndrome and the T-1000 fallacy.