Hello good sirs of the internets.
This post is reluctantly accepting as a premise of it's proposed solution, that the doomsday-sayers are right, and that mining may at some point become unprofitable.
What I suggest is, that a independent organisation is set up, with the sole purpose of mining BTC.
The organisation will be non-profit, and all proceeds that it creates from mining, it will use to keep itself running. In other words, it will be self-sustainable as far as possible.
This also means, that as long as mining is profitable, this organisation will hoard funds, since it will spend less that it will have as an income. These funds will then be used in the event that mining becomes unprofitable, where the organisation will then continue mining at a loss.
It will have to be started from donations ofcourse, and it's probably not very realistic or do now, but rather something that can be done along the way, if BTC starts becoming a serious world wide currency with billions of dollars of value in it.
The point of this post is simply to give an idea of how the problem, that may or may not exist down the road, can be handled.