Well at least 8K scales up 4K pretty easily?
Like with 4K, you can scale 720p, 1080p, 4K, and with 8K now 480p as well perfectly with nearest neighbour.
The sizes you are looking on these sets being 70"+ kinda makes upscaling no bueno though. Interpolation is getting a lot better now however.
The main driver for 4K and now 8K is that home TVs are getting larger and larger on average YoY. The average US TV is a 55" model now with 65" being really popular as well. At those sizes and at typical distances 4K makes a ton of sense, and if the trend continues and 65" sets take over 8K starts becoming attractive as well, especially as a reach from a 65" 4K set to a 70-85" 8K set.
One big issue TV manufacturers are facing now though is the US market has started massively diverging from the rest of the world, mainly due to space.