You're confused. When a gold miner pulls gold out of the ground (or out of the ocean, or off an asteroid), everyone else's percentage of the total declines. That doesn't happen with bitcoin. My one bitcoin is 1/21m of the total, forever. The two are not the same.
There has never been a fixed supply money, or ledger, or whatever you want to call it, because fixed supplies don't exist except as a mathematical construction.
This use of a mathematical construction as a monetary base is an unprecedented experiment. Whatever you think will happen or should happen is unproven speculation.
If you're talking about 100 years from now, sure, that's a new thing. But the idea that this new thing is of any relevance stems from the idea that money supply matters at all. Before that can be seen as any kind of concern, there needs to be a reason why that might be a concern. Otherwise it's no more remarkable than the fact that it's unprecedented that a money system is named "Bitcoin" or that it starts with a B.