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.
one day miners will be difficult to produce bitcoin because as I know when bitcoin is getting closer to supply then the level of difficulty to break blocks that can produce bitcoin will increase even the result of miners will be drastically reduced, so the supply limit of bitcoin will have an impact on price and life bitcoin in the future.
because when bitcoin becomes more difficult to obtain there are 2 choices, humans can still use bitcoin but with very high transaction costs or humans don't use it anymore because bitcoin prices are too high only that is likely to happen in the future when bitcoin has reached the maximum supply limit .