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 mined all, the fees will raise more 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