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.
I think it will be more than 19m that will be in circulation even while excluding Satoshi but I wouldn't assume it is lost forever, anything can happen, we are not the owner of the seed phrase neither are we in his/their mind.
While these said bitcoins tagged as being lost/dormant for too long, some of them are just not being access by the owners still waiting for a time when they may decide to move it, we saw things like happened this year. Also, some lost coins will also be found while a few will still lost theirs but as the value continue to increase the rate of losing bitcoins will be lower and the rate of even finding lost ones will increase well enough. It will become treasure hunt on the internet and the most valuable treasure will more likely be bitcoin.
Na true you talk we no fit to accurately say Satoshi stash and other wallets dem wey don dey dormant for a very long time say dem be lost Bitcoin. Dat theory na just speculation im be, so far we no get proof say di owners don lose dem seed phrase, dem fit to surprise di world by moving di Bitcoin. As e be sef treasure hunters fit to channel dem focus to untouched massive Bitcoin stash for many years and e go be finders keepers for dem.
As for di 21 million Bitcoin I de sure dem go mine everything because e don de fixed so di only ones wey go miss na di ones wey don become donations for Blockchain.