Is the 21M BTC supply promise realistic?
It's for real and it's even recorded and programmed to have that supply limit. Reaching that max limit will take around a century or more until there's no more bitcoins left to mine but it's all going to be in the circulation.
Miners will continue to mine for the same thing and that's for the confirmation of our transactions. I'm also confused what you're bringing up here or what bothers you about the supply limit.