Why? The whole point of that rumored PS handheld is to give an option for markets & demographics that prefer a portable experience. As long as it doesn't hamstring what games can do with the PS6, it'll only be a net gain to the ecosystem.
It'll help them out in regions like Japan, and it can act as something with native local play of PS5 and (select) PS6 games (plus PS4, obviously) while having room still for another PS Portal (which would be cheaper, since it'd be streaming-only). The PS handheld just needs to be based on shared technological features with PS6, i.e next-gen PSSR, CPU & GPU architecture/instruction sets, SDK API support etc.
A very different situation from what Xbox is going through in terms of hardware.