5K is fairly specific. At the moment is found only on some computer monitors, and it seems to be, that that's where its going to stay.
8K seems to be whats emerging as the next big jump in ultra HD. But it seems to me that filming in such HDs seems kind of pointless unless you have something to display it on.
I would hate to think how much an 8K tv costs at the moment. (After a bit of research, I've found that there are no 8K tvs readily available at the moment, but when the first ones do come out, you'll be lucky to find one under $20,000US)
The problem is that 4K is still fairly rare when it comes to available content. So making the jump to 8K would be fairly redundant.