As long as there are things which can only be purchased with bitcoins they will always be relevant. If people can only buy drugs on the internet in bitcoins then bitcoins will continue to be used.

I could possibly say the same about USD. As long as I must pay my taxes in USD the USD will always be relevant and continue to be used. There might be other things which also help to make the USD relevant, but this is its basis, its keystone point to being.