PoW miners tend to spend more and more resources on finding blocks, until the cost approaches the rewards. But the rewards go up as the cryptocurrency becomes more popular, because the price and transaction fees go up. Therefore, a PoW cryptocurrency tends to "eat the world" as it becomes bigger.
That's why Bitcoin is already approaching 1% of global electricity consumption, if it hasn't passed that point already. If the price were to go up tenfold, then so would electricity usage (roughly). That's not sustainable, both technically for grids and economically because electricity prices go up.
Because of that, I foresee two possible futures for PoW cryptocurrencies:
1. The resource usage overshoots and PoW collapses because it gets banned everywhere. (This seems to be playing out now with China having banned crypto mining, Kazakhstan running into grid issues because of the miner influx, and Sweden arguing for a ban in the EU.)
2. The popularity of these currencies stops growing and only some niche applications remain. Speculators leave because there's no more money to be made. Prices go down.