If Bitcoin replaces fiat, then everything will have eventually be priced in bitcoin, which is "currently" capped at 21M.
It doesn't mean nothing else than Bitcoin will have value, but the double coincidence of wants insure that Bitcoin will be the widely preferred unit of value. It is then just natural that the value of all goods will trend toward 0.00000001 BTC, both because Bitcoin is the best money AND because of its supply cap.
I don't get your point about "Have you even noticed guaranteed deflation through a centrally planned rate of supply didn't work, at all?".
Are you saying that it never worked, or that it won't ever work?
I also fail to see a relevant point in your remarks re the centralization of Bitcoin Core or WordPress Core.
The 21M number is symbolic. As mentioned above, it's just a simple way to get the trend of goods priced in Bitcoin in a hyperbitcoinized world.
Lastly, even though the 21M supply cap isn't mentioned in the whitepaper, Satoshi mentioned it several times over the years... and what he inscribed in the genesis block speaks for itself.