Yeah, the cap is fixed and never increasing. I think the point OP is trying to drive home is that even though all 21M coins have been mined in the decades to come, we will never have full 21M in circulation because some of the supply has been lost aka inaccessible. I estimate only around 19M coins will be accessible even when all coins have been mine. We can assume that the satoshi allocation is lost forever.
But as we were told, that all lost coins have been recircled back to the market price as spread, believing that all bitcoin lost due to wrong address have been believed to have returned back to the network, but only what I don't understand is this, how they will be able to differentiate between lost asset and a domant asset that have been on hold over time.
Also, as you have contributed, we are going to have all the 21 million cap mined completely and after that, even if there is going to be a lesser amount reducing from its total supply expected to be in circulation, then it won't be something much to right about about, maybe only the 1 on top of the 20 million total supply.