Miners can mine transactions, and get the fees.

I am sure that bitcoin will be forced to increase the size of the unit. As the popularity of bitcoin will rise will increase the number of transactions. Easy to see how bitcoin will become helpless in competing with such upstarts as the bitcoin cash. I think that the developers should pay attention to the fact that the miners could not dictate the conditions and slow down the development of the whole community. If nothing changes, bitcoin will not live 120 years.

It is terribly tough to have this conversation because we won't know what the block size will be, how much one bitcoin will cost, and many other things (amongst if bitcoin will be alive then). Therefore I think this conversation is essentially moot because of that but it is nice to discuss theory sometimes I guess

Satoshi's original vision would be that after 120 years, miners would be essentially located only in big, corporate mining farms which we are starting to see the birth of today. There are lots of people mining in warehouses in cold climates to keep their rigs cool without additional resources and ultimately even people trying to buy properties condisuve to mining such as to build hydroelectric dams and whatnot. It really is quite amazing how far the industry has come in such a short time spam of under a decade.

You seriously think you can accurately predict what the economy will be like in 120 years? How will the natural resource scarcity and overpopulation of the earth affect the economy? How will the imminent automation of the workforce affect the economy? No one knows. It is wishful thinking to assume that bitcoin will still be around until 2140.

I guess when it comes and the supply bitcoin is ended (no rewards)
miners can only mine transactions and get the fees from confirming transactions.

After that twenty years, when it is supposed, the last bitcoyne will be produced, if it still exists, it will change almost completely. Strictly speaking, I do not think that in such a long time, the mining itself will remain unchanged. By that time, the very digital currency itself will change very much, so it is pointless to think of such a time ahead.