As a matter of fact, you can also anticipate the point when the amount of block reward is really really few and there's tons of users using bitcoin. That would be a disaster when the lightning network still hasn't been figured out. The miners would still be there, supporting the network. "But why would they do that when they receive very little to no rewards at all?" Because they have bitcoins, so they are going to continue supporting it. By that time, they probably have already made fortune and continuing their mining rigs to run will not be that expensive for them.
Plus, if they stop supporting bitcoins, it's them who will suffer the most since they have the most investments.
If bitcoin has become the international currency in future. Possible bitcoin prices have stabilized and are only ready to be used. What persists is traders in market, because miners will be significantly reduced.