bitcoin will never be able to suck all the fiat money, they can just print more without problem
also fiat is already worthless, but you're obligated to use it for the majority of things
if you mean collapsed, i don't think so
It's not worthless simply because people accept it. If someone gave you 1 million dollars you wouldn't say "no thanks, this is worthless". So that's the point. Even if the system is rigged, people all over the world recognize the dollar, so as long as people recognize it and use and accept it, it's going to have value.
When BTC is 10K each, it just means you can buy the same you would be able to buy 10K dollars, that's all.