It is true that the supply of bitcoin is only limited for about 21 million but the supply of other cryptocurrencies is not. There are many other cryptocurrency out there that have the potential to grow more and develop. That may happen in the future and may surpass the capability of bitcoin. In that case, we can still earn money through cryptocurrencies even though if the bitcoin's supply will run out.
The key question, to me, is who will have power in this system. The more types of altcoins survive, the more power the elites have.
You don't know which cryptos to invest in. The elites do. Whatever they promote (by buying) will do well.
The more this happens, the more the crypto-based system will be like the 'fiat' system.