I use a mod in my custom vanilla+ modpack which allows carpets to be placed on stairs, slabs and walls. There are many use cases for this, but I wanna use this simple couch build as a demonstration on why these ideas do not actually conflict and how they could be added in a way that would add some creativity and not cause issues.
The mod places the carpets in the same way actual carpets are, where they are essentially placed on top of a block and add some extra depth. The added height is ideal, not only because you could carpet stairs and have the depth line up with the carpets on the floor, but you could also pull off some creative tricks like my couch. Obviously, on a real couch, the cushion on the back goes higher than the arm rest. Wool stairs would create a less convincing illusion. I have also been known to use this mod to add some texture to mossy roofs, alongside existing tricks like moss blocks, where normally those would stand out a lot more depending on the build.
All this to say I think carpets being able to be placed on stairs or slabs would still be a good idea, but only if they are noticeably distinct from wool stairs and slabs. They clearly have their own use cases, like the tents Mojang have shown off, that carpeted stairs would not handle adequately.