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?
That is why you need to write it down in a very detailed manner, so your family can gain access to it and it would not go to a waste, or else it would be irretrievable for anyone. Yes, the bitcoin left on the market will be decreased if the bitcoins can not be recovered by anyone, but it will make the price higher as the number of bitcoin left goes down.