When each of the 21,000,000 Bitcoin have been mined (in over 100 years) the miners will still be rewarded with the Bitcoin they make verifying transactions. When you transfer Bitcoin you pay a fee to the computers that verify your transactions. They are doing the hard work making sure you have as many coins as you say you have, so they are rewarded.
Bitcoin’s transaction fees are significantly higher than other cryptocurrencies, but people still use Bitcoin because it is the “brand name” of cryptocurrency.
Will mining be worth it when there are no new blocks?
Absolutely. If the number of mining computers decreases, then those who keep mining process/verify more transactions and get more fees. This is a sort of equilibrium that always ensures that mining is profitable. If there are too many computers connected to the network, then the fees will be less which causes many to get discouraged and stop mining.
All of this assumes that people still show interest in Bitcoin in over 100 years. It’s crazy to think about a point of time so far away. Bitcoin (along with other coins) might be powering society as we know it. Or perhaps it crashes completely and other cryptocurrencies take its place (this is more likely, but not any time soon).
Cryptocurrency is amazing because it does so many more things other than hold value. There are softwares built on cryptocurrency and blockchain technology alone. That’s why I think that the “What if Bitcoin is all mined?” is a silly question to ask, because there will be other coins in existence that solve all of Bitcoin’s problem.
I believe that cryptocurrency technology is the future as we know it – as big as how the Internet changed the world. It’s best to do your research now!