What happens if all bitcoin (21 BTC) has been mined?
This is the answer
Are miners going to mine alt?
IMO It's their choice. When bitcoin is all mined, the fees will increase and they will have a lot work of transaction confirmations. If the fees can make they earn profit, the miner can still stay with it.
or supply will be added?
No, it's not bitcoin anymore... it's fiat currency, where the supply is unlimited
Haha. Lol. A lot of thread that was made already in forum. I just don't no why there is still existing a thread like this. It was already a topic even before. If there is no more bitcoin to be mined, the miners will decreases or be stagnant in their mining of btc. So, in order for them to be profitable they will increases the amount of fee for transaction. And probably, bitcoin price will increase.