While it's true it will eventually re-enter, there's no danger of that for quite some time, as Hubble operates at a very high altitude, about as high as the shuttle can reach, in fact. This last repair mission will install hardware to allow for future controlled de-orbit.
As for selling it to a private interest, I don't see that happening. This would require science customers to pay for operating it without government subsidy, which is a drug that scientists and universities are addicted to in this age. And when would NASA sell it off? Before it needs more repairs, or after? And while it's one thing to operate a functioning satellite that is already in orbit, it's a whole new boatload of money to pay somebody like SpaceX, etc. to fly another repair mission.
So this is it: Hubble gets one last repair and when it breaks again, it stays broke until it hits the ocean.