I do think there is value for macOS long term as there is an obviously place for Mac’s, but they already are a relatively small share of computer users. iPad’s sell more than Mac’s as it is. A few small changes and Apple could shift a good bulk of their users to iOS. It makes better business sense as it’ll lock users into a closed controlled ecosystem. I think this is what Apple wants long term.
Yeah it depends on someone's usage. I think that for people that used to use their computers at home for watching movies iPad is a great replacement. So that category definitely is already filled.
We know that the iPad plays quite good role when it comes to photo and video editing. Digital art as well so this is where people can use it professionally. Another group is music editing as of last I checked.
There are three categories that for now I am not sure that can go only with iPad:
1. Heavy Microsoft office users (they will most probably need the desktop versions of the software plus bigger screen plus mouse/trackpad)
2. Some software developers - that might change at some point with Development going more to virtual machines and servers and iOS improving but for now it's still not the case. Though Developers mostly need either a lot of virtual servers or workstations and desktops.
3. Some really really really niche cases. For example in university I used 3D Max studio to run OpenGL scripts to render designs. That is however really really specific case.
I can't come up now with other cases. I honestly think that the Microsoft thing is more like a blocker. Honestly we still use enough Microsoft programs and the moment we do, we need the desktop versions.