8K in 3:4 ratio is 45MP. CFast cards writing speeds are, AFAIK, just shy of 400MB/s. So lets assume Canon can make a camera that shoots 45MP at 10 fps.
What would be the usage scenario? Sports photographs cropped 50% and printed to cover a wall?
The Photo Gallery/ Print shop/ Custom Framing shop where I work/ essentially live handles prints in excess of 4ft x 8ft. We regularly print/ produce 4ft x 6ft canvas wraps and have done about 80 Acrylic Facemounts from 4ft x 5ft. to 4ft. x 8ft. We were looking at doing a 4ft x 18ft but are a bit held back by material sizes.
Having extra resolution to crop an image and still be able to produce large prints is pretty amazing.
So yeah, the more pixels the better. The work is out there, just because you're not getting it.
Currently, the owner/ photographer of the gallery uses a D850, previously a D800. Personally, I prefer Canon, so I use a 5DMIV and going up to 30mp has been amazing for me. Though, the largest prints of my own work that I've made/ sold has been 24" x 36"