Forgotten bitcoin in a wallet is not lost but rather it will cause a decrease in it's supply there by increases it's supply there by giving it more value. But this doesn't say we should all forgot or lost our private keys for this to happen. It is also required that bitcoin must also be moving around just like fiat does and not just stay stagnant in a wallet doing nothing.
still it is Lost , because none can actually access the funds though you are correct and this will add value to the remaining coins and decrease the remaining volume .
actually like what Satoshi says(According to Loyce quoted post of satoshi) this will be a donation to the other Holders because surely the price will increase and this will favor the investors and bitcoin owners.
Benefits of lost Bitcoin to every holder is that the supply is reduced even lost Bitcoins cannot be accessed anymore . Hope it can somewhere be noticeable once a lot more people have been involved in crypto.
It is noticeable , because now that we are almost done mining every bitcoin(actually long years more but small amount remains) sure the price is not in increasing scenario.