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
Except that it's neither known that he owns that much, nor that they're gone forever. They're part of the UTXO set, so they're definitely not gone forever. It is possible to spend them, and it's likely that they will be spent sooner or later, because the ECDLP will someday be solved. And early coins were sent to public keys, not to hashes of those public keys.
I understand that BTC can be "burned" by being sent to an address with no known private keys, and without access to private keys, any btc will be lost forever.
Again, not forever. Even if you throw away the private key, there are about 2^96 other such keys that you don't know, and can unlock that output. If you want to provably burn coins, you should use
OP_RETURN.
This leaves me with a very hypothetical question, what if all btc is burned (or lost)?
We'll move onto other currencies. But that implies that every bitcoin user is completely irresponsible, which is not the case.