Taking back the lost Bitcoin to add it to the supply wouldn't be achievable because if they do that that means our coin ain't safe because definitely they can access anyone's wallet and claim their Bitcoin as well and moreover, do you know if they actually lost of forget their seed phrases of the wallet? because some persons can actually buy Bitcoin with the intention to leave it in their non custodial wallets for a very long interval of time without accessing it and it would now look as though they've forgotten their seed phrases and any attempt to either add or subtract the quantity of Bitcoin remaining to be mined will become an error in the process.

A lost bitcoin is a lost bitcoin as no form of recovery without the seed phrases would be obtainable so it would just be stucked there forever after all the owner who lost control of it also bought it with money so if he loses it then it's actually his brunt to bear that's the more reason why Bitcoin is trustworthy because it can never be manipulated unlike fiat whereby if something of such occurs they can maneuver and manipulate it, so it is important to keep seed phrases safe and secured and make an offline backup so that in a case of losing it one can easily recover it. Moreover, what of a situation whereby the owner dies? So no one can inherit the coin because that is what stands bitcoin out as there is nothing like a mediator to ones assets.