Will the total supply be increased because some bitcoin holders have died and others have lost their private keys?
You can't know that bitcoins are gone for good unless you've made an OP_RETURN transaction that essentially proves the burning. Even if you could, the system is designed to be deflationary and as long as the community thinks it should be 21M, it will. No one will ever come into an agreement for this, though, because no user is benefited from inflating their currency.
I have said this before, I believe when it comes to “life or death” of Bitcoin, the community will come to consensus to a hard fork to save the protocol, and agree to some measures to continue the sustainability of the system.
What kind of hard fork will that be? You mean if the security drops in the long term, right?