Coins are 21M, although there is no fixed number for blocks. So after the last bitcoin is mined, transaction fees will be the reason of mining. I believe that the average fee will go too high in 2140, but if you think it clearly, even on 2080, the generation of bitcoins won't be anything big. So fees will play big a role then, too.
The issue here is the limit of coins, if we move from PoW to PoS that will not make bitcoin have more than 21M coins.
How exactly can you force everyone to move from PoW to PoS?