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.
There has never been a money supply that has grown at a constant predefined and unchangeable rate, without regard for monetary demand (another mathematical construction that doesn't correspond with gold). That is happening now, not 100 years from now.
But even things that happen 100 years from now can affect the present (or near future), by discounting of expectations.
As for why it is a concern, it is simply that it is structurally different in an unprecedented manner. Again, you may thing it is the greatest thing since sunlight and rainbows, and it may turn out that way, or it may not. Bitcoin is an experiment in progress, and multiple outcomes are possible. If you claim otherwise you are dishonest or deluded.