Let us assume, that there are only 10 people on earth and there is only 1 Bitcoin.

If it is equally distributed, then everyone will get (1/10)BTC or 0.1BTC

If the population is 10^2 or 100 people, then everyone will get (1/10^2)BTC or 0.01BTC

.
.
.


If the population is 10^8 people, then everyone will get (1/10^8)BTC.

As Bitcoin does not have any physical representation, it can be divided up to any extent. At this moment, the protocol supports division up to (1/10^8)BTC, but, if required, it may be divided more.

So, wherever the population goes after 2140, it wont be a problem to use Bitcoin. Just units for daily usage will change from BTC(10^0) to mBTC(10^-3) to uBTC(10^-6)...