I guess because by that point the price of electricity to mine will become so huge no one will want to, and the psychological deterrent of buying insanely expensive coins. I can see it crashing because people will think "well there's no more supply, and those that have it won't relinquish it." I don't know there's an economic term for it, something like fatigue.
Miners will still be able to mine bitcoin through transaction fees. And yes, the distribution will be exceedingly low, but the assumption(and what we hope, I guess) is that bitcoin at that time will be priced a lot higher from here that miners will still be incentivized to turn on their miners. No guarantees that this will be the result in the end, but time will tell.
But also like... Right now we're still in the early stages, and Bitcoin isn't really being used as a currency.
Like seriously, this argument has been used time and time again even though it's simply straight out false. People are using bitcoin to pay for some services online(mostly vpns/proxies/servers/giftcards/etc), in the gaming markets(Sythe.org), the recent El Salvador, etc. Sure you could say that adoption is low, but "
isn't really being used as a currency"? Straight out false. Not because you don't see bitcoin being used in your local coffee shop it doesn't mean it isn't being used as a currency.