The home console tech ceiling is coming. There is only so puch powr you can squeeze into a under-the-TV box before something has to give. In the case of the PS5 it was actually fitting it under a TV in the first place. How much cooling space will PS6 need?!
Handhelds therefore offer a quality sidestep for the industry. There is only so much a person can see on an 8" display so PS4-level graphics are really all that is needed. Customers are happy to trade off graphics for battery life (See: Gameboy vs Game Gear) and phones don't have buttons. Well, gaming ones anyway.
If Microsoft are going for a dual-console strategy again it makes more sense to make the lower powered model a handheld. I'd buy one tomorrow if it meant being able to take my Xbox 360 collection on the go.