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?
Nothing ever gets lost. People are even talking that quantum computers will hack bitcoin network and who cares about lost bitcoins, they should be easy to restore. I wouldn't say that it's something to laugh on because for example in past first hard drive had size of 3.75 megabyte while nowadays the max size went up to 16 terabyte. Can you see how dramatically it increased? So who knows how soon we will be able to crack some things in near future. Bitcoin won't remain the same, it will have a need of security development, adding of some new features and some modification in overall. Maybe we will get all lost bitcoins but who will own them?
Nothing is impossible, even reviving of lost bitcoins. It's just a matter of time.