Everyday people lose access to their wallets, send Bitcoins to wrong addresses. It results in Bitcoins being lost forever. In the meantime, there is no way to prevent it.
So, lets suppose that Bitcoin would exist forever. It means that at some point there wont be any coin left in the network
I know there are other implications/discussions about Bitcoin supply. But Id like to limit this topic to this concern (Bitcoins lost).
Is the 21M BTC supply promise realistic?
I don't think your premise makes sense.
Other Bitcoin users who haven't lost their bitcoins have a strong financial incentive against diluting the money supply. As a matter of fact, the supply cap is probably considered the most sacred of all Bitcoin consensus rules.
Now that bitcoins are so valuable, we can also assume they are being lost at a significantly lower rate than years ago. I don't expect every bitcoin to be lost; not even close. So, I doubt this scenario will ever come to pass.