Nope, this is no existential question (although...). Everybody's talking about "When will Bitcoin die", but what if you die, and your family don't find your key, or don't understand it, or think it's only a useless sheet of paper with some code on it, or simply throw it away... Does this mean the currency's gone forever?
I find the possibility of this event rather realistic (yup I don't believe in eternal life), so that means that someday there might be 21M BTC but only 18M (and counting down...) is left in the market?
There is a feature in bitcoin called nlocktime, that basically acts like a dead man's switch. So it's basically a raw transaction to an specified address, the only difference is that the transaction can only be activated when a certain block height is reached. For instance, the 1,000,000th block. If you are still alive then you can obviously get your wife or whoever to send your coins back, but if you die(awww) then they get to keep it.
But you're right, this feature isn't widely used by any means.
So long term supply of bitcoin should be going down especially after the block rewards become miniscule. The number of burned coins, forgotten passphrases and inactive coins aren't going to come back to life unless an extradinarily large amount of collisions were found, which is pretty much impossible.