> Yes, humanity could theoretically collectively decide that from now on gold is not valuable any more and chose some other scarce material as a way to store wealth

No it's not going to work because gold is very scarce and extremely valuable as a material in any case.

Yes, the price would take a hit, likely something between silver and current gold price, which would increase use and demand and then price again.

There's a reason we're thinking of mining it from asteroids.