So when mining we actually calculating other people transactions. What if everone just mining and nobody make transactions? The whole point of bitcoin is that it has no fees if you use professional mode wont that defeat the purpose of not having a central computer? And after 2140 there will be no reward to maintain the network then why would anyone bother doing it? Would the people investing money into these servers not just quit right away causing the whole system to collapse and the value of money be completely lost?
Bitcoin is the number one cryptocurrency and its market cap is similar to market cap of top companies. There is no way for such a popular thing to not experience transactions on chain but even if we imagine the scenario when nobody makes a transaction, then I can confidently tell you that there will be nobody to mine. It's impossible for everybody to mine and nobody to make transactions because both of these scenarios are the opposite of each events. If nobody makes transaction, then why will somebody mine something that's dead? And if everybody mines, then it automatically means that many people make transactions.
The whole point of Bitcoin is not 0 fee, if it was, then there wouldn't be a fee system. After 2140 there will be no reward but Bitcoin will be so popular at that time that there will be millions of transactions every day and that will be enough to motivate miners to keep their mining rigs turned on.