I think that you have got many answers from the other members here, better to use the search feature in order to get early information about whatever here in this forum.
In every article it is mentioned that bitcoin is only 21Million BTC to be mined.
Yeah, it Is only 21M BTC. There will be no additional supply, it has been fixed that BTC is limited. However, never worry about it when you are going to mine. The last BTC to be mined maybe next 100 and more years. And at that time, we all here right now may be no longer live. So, better to focus on what we are doing right now in the crypto world.
Are miners going to mine alt?
We do not have any idea about it because it will happen in next hundred years probably. So, we do not know whether this crypto will still last longer or not. Or even what kind of altcoins will be there.