As I understand it the PS4 CPU was shit. It was so inadequate that the PS3 CPU was better in some aspects. It would be like comparing the Series S GPU with the One X. Only better because is was more modern. So a lot had to be done to use the GPU instead of the CPU when possible. And the PS5 had to be able to use all the tricks that devs used to gain performance on PS4 games. AMD said that their offer would work. But making sure of it took time. The PS4 Pro had to be compatible with the PS4, and the easiest, cheapest,fastest way to do it was to take the same CPU. For the same reason they just put 2 PS4 GPUs for the PS4 Pro.
I trust Sony to do better this time. I think that they planned the PS5 Pro since the conception of the PS5. And that this time their partners also have plans for it. The PS6 is so far away that maybe they will switch to ARM, maybe they will put a Zen 2 for that like they did in the PS2 for the PS1 backward compatibility. Of course we need to wait for the official reveal to know more. The more hype they do for it, the more confident they will be IMHO. But I am personaly not asking for much. Better image quality, having their performance mode as good or better than base PS5 quality mode, some quality of life gains like more RAM for system use.