You initially operate with an inaccurate figure of 19 million BTC. Immediately you need to subtract ~1 coins, which is stored on Satoshi's wallet. I can't tell if access to these facilities is lost forever or if it's just being stored, but at the moment, this BTC's don't participate in the circulation for sure. Next, you need to subtract the number of coins, access to which was lost forever. For example, due to a breakdown / disposal in the HDD trash or due to sending BTC to the wrong address, which also leads to their loss forever.
They are lost at the moment like if you forget the seed of your wallet or password of the drive there are still chances that they can be recovered and brought back to circulation but we should assume the only lost one's which have been burnt through OP_Return command as they are invalid now.So there are approximately 4 million coins which can be back into circulation if the owner found or remember their password/seeds of wallets.
But yes if people are careless in backing up their funds with security then supply will initially reduce as there will be very few mined and it will take another century to mine the remaining 2 million coins approximately.
That "lost" coin logic is a bit skewed though. Meaning, if it wasn't in circulation, it didn't matter to the public anyway and it didn't changed the price anyway. Meaning, if there is 1 million bitcoins that have been with someone until today, and that person exists (not like satoshis 1 million) but they never put it in the market, then if that person loses it, that didn't change the price at all since it wasn't on the market anyway.
What it does is that the fear of that person selling is gone now, that's it, and that fear may cause some people to invest more clearly and trust it more, but that amount wouldn't be too much so it won't change the price all that much.
