Another way to think about it is that very few games are using 100% CPU capacity and 100% CPU capacity at the same time. This model gives the developer a combined compute budget, which can be allocated as required for the task.
Right, but to the GP's point, for those games which do aim to max out both at the same time (or at least get as close to that as possible), this could be a development pain point unless the developer has control over which gets prioritized. Once upon a time this sort of maximization was a common goal for console game development, though I don't know how true that holds nowadays.
But I think this style of hardware also changes the way you make optimization decisions. I.e. in a world where you have a fixed CPU/GPU budget, you might try to think of ways too move work to the CPU if for instance your GPU is saturated. But if the relative capacity is variable, you can just do things in the most efficient way overall, and you just have to make sure you don't exceed your overall budget.
The only way I can see this going well if this balance is fixed by the developer. E.g. the developer specifies I want 60% of the power budget to go to the GPU and 40% to the CPU. If this is handled dynamically by the PS5... oh boy.