I don't really follow. It doesn't matter if Zen 2 or Zen 3 is the baseline and it has little to do with Series S being the min spec. The question is whether these consoles would have seen meaningful improvements from Zen 3. And my argument is that they wouldn't because the difference is too small.
And this applies twofold when you factor in that most games are going to be GPU bound anyway.
This is what you said: "But my point was that a game running at 21fps won’t look very different from one that runs at 30fps. The difference is too small to be meaningful.
Remember that 33% is also the difference between the PS4 and PS4 Pro CPUs. It never amounted to much other than slightly improved draw distances and the difference between Zen 2 and 3 would likely have been smaller than that."
1st point: No, there is actually a big difference between 30fps and 21fps, 21fps is below the industry standard of acceptable fps.
2nd point: 33% Faster CPU is a big deal. Those gains are so good you don't expect new CPUs to be that much faster then the CPUs they are replacing. Think of it as simply as getting your console to do 33% more work in the same amount of time.
3rd point: You say the Pro's CPU gains didn't amount to much over the base PS4. To which I replied "your missing one key component. Last gen games were designed for base consoles as lowest common denominator so any additional power of the Pro and One X would only translate to higher fps". So unlike GPUs you can't just scale CPU tasks to do more because there's a more powerful CPU under the hood all of a sudden. Why? "The reason for this is that the work the CPU does(world simulation, physics, AI etc.) can't scale with stronger hardware because changing those means fundamentally changing the game(imagine a Mario game having different physics with more powerful hardware). So the easiest scaling thing is fps which is what you saw last gen with the One X."
The work the CPU does can't scale anywhere as simply as the work the GPU does. Higher resolutions, more particle effects, higher quality ray tracing, better antialiasing, higher resolution textures, higher quality shadows all of these easily scale and do not affect gameplay but scaling the AI, the physics or the simulation of the game world would change the fundamental gameplay of a game. This is why even if you have a stronger CPU running the same game most of those things can't be changed or improved. Somethings can be like number of pedestrians but most things are too complicated to scale them with a stronger CPU.