They have said they want to keep a fairly long-term performance target for game devs optimizing for the device. Consoles do the same thing. Another part of that is improving margins over time.

Component cost, engineering cost, and it would change the performance target which could cause some devs to leave the old one behind

None of the currently available RDNA3 APUs are good within their 5-15W target. They could custom design a smaller one but that would increase costs a lot and the perf benefits aren't that big when still limited by LPDDR5