Hello 
I understand that BTC has a hard limit of 21 million Bitcoin, and there's roughly 18m BTC's that have been released so far.
As already pointed out by @Charles-Tim that there are more then 19M BTC that have been mined till date and it will take another century for the remaining one so it's still a lot of time.The only way bitcoin can come into circulation is mining process. I understand that BTC has a hard limit of 21 million Bitcoin, and there's roughly 18m BTC's that have been released so far.
I also understand Satoshi Nakamoto owns at least 1 million BTC that is assumed to be gone forever, and there are many other wallets with massive amounts of btc that have seen no movement whatsoever in 5+ years, leading to the possibility that those coins have been lost forever as well.
Those coins you are talking about are not lost actually it's just that they are dormant from many years or say people have lost keys and password to their files but if they have access to it the coins come back into circulation.There are many users who found their wallet.dat files or even their hard drives after so many years and coins supply increased.The only way the coins lost are when they are sent to OP_Return command where they are burnt forever and this decrease the total supply of 21 million and other reports that suggests millions of coins have lost are in probability of coming back but not the one's which have been burnt.
This leaves me with a very hypothetical question, what if all btc is burned (or lost)? After all 21m btc has been released, the number can only go down, right? I understand it would take an extreeeemeeely long time, but could humans run out of btc after enough burning? What will continued btc burning do to the economy over time? Is this something that Satoshi Nakamoto considered?
In Satoshi words to explain your doubt : Quote
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.
Dying bitcoins
Dying bitcoins
I don't see any possibility when all the coins will be lost and if it happens what will be the usage of it and we will remember it a currency of past but it's the future and people have taken the backup seriously so nothing like that is going to happen.But if we are talking about century then nothing is so certain and be optimistic about it.