True, 60fps aren't guaranteed, some games will get a new 120fps mode instead.
But yes, some games that were capped at 30fps will continue like that when getting a PS5 Pro patch: particularly when running in a new 8K mode.
Its up to the developers right?
Yes, devs can do whatever they want with their games. But if a game that is capped to 30fps in PS5 gets a PS5 Pro patch pretty likely will be to give it a 60fps mode.
If it stays at 30 pretty likely would be in a 8K mode.
The only case where I could see it would be in a game that runs a big ass open world with super high end visuals way beyond what we saw in games like HFW, using also RT and real time global illumination. But I think we won't see this during this gen.
Because these games pretty likely would get a 60fps mode without RT and RTGI and lower resolution in base PS5. So would be 60fps too in PS5 Pro, or even 120fps.
I think the typical case of a new PS5 Pro mode will be -for games that in base PS5 have a 30 fps quality mode and a 60fps performance mode- a mode with better image quality and resolution than in the quality mode running at 60fps. And optionally, depending on the game, to get a new 120fps mode and a new 8K mode (30 or 60 fps).