They can only control the orientation if the gyros hold out
Once the number of functioning gyros drops below a certain point, they'll no longer be able to control the HST's attitude. Still, I don't think it'll pose a serious reentry hazard. The orbital inclination is about 28 degrees. The vast majority of the Earth's surface between 28 South and 28 North latitude is water. I suspect that most of the HST will burn up on reentry. Skylab and Mir were in much higher inclination orbits and were many times more massive, but the ground controllers still had attitude control to help guide the reentry to unpopulated areas.