The thing is you only see large IPC boosts when the architecture is new. Especially for a ryzen - AMD wanted Ryzen out of the door ASAP to bring in money - that left a lot of places in the architecture to improve - that's what we've seen with zEn+ and zen2. Zen3 brings more tweaks. But I could also believe they are running out of big things to change. We saw this with Intel - they brought out Skylake and Intel was able to tweak some things for a couple generations and after that they had done everything they could and it's flatlined every since.
Now Zen4 May only have a small IPC gain but AMD is making another change which will really help workload performance and that's 4x SMT multi threading - so a 16 core Zen 4 shows up as 64 threads in Windows - so even if Zen 4 has no IPC gain that along will offer a good generational performance gain
I think you hit the nail on the head with your earlier assessment. Intel and AMD’s tick tock approach sounds to me like Fudge-correction. Although I would argue Zen 2 has a bit of both. Ie going from 12nm to 7nm. Zen 3 is meant to be the refinement stage thus all the previous 2 years of design flaws to be ironed out.
Zen 4 is then a new fabrication 5nm and thus new architecture. I think it will bring efficiency on Zen4 and some small IPC. It won’t be Zen5 and beyond you get the big IPC jump. Zen4 should be on DDR5 and PCIe 4 and beyond. Otherwise it will be pointless. AMD may well overlap Zen3 and Zen4 to have DDR4 and DDR5 market cap.