The lost bitcoin is part of the coin in circulation as it will add to the marketcap. But because they are lost, this will increase the value of bitcoin because it will make bitcoin to be more scarce.
That's actually true, lost coin can not be recovered but it add value to the market. Because one good quality of anything is scarcity, for anything to have value It must be relatively scarce. this was a quote from Satoshi Nakamoto about lost coin.
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.