> in regions with cheap hydro power, the miners can use so much power they end up making power more expensive for everyone.
Not if there are laws to keep electricity price within a range. In places like Canada electricity is not necessarily traded in public for-profit markets, and must by law stay below inflation. I agree that in can be a problem in (most of?) the US.
> I really wonder what happens to Bitcoin (or any PoW coin) when it no longer becomes economical to mine new coins.
That might be the equilibrium price, the electricity/production cost. Isn't the pow algorithm smart enough to keep rewarding at least above electricity cost?