This is exactly right that there are many facets. It's not so much arguing whether tyre or the axel is more important, as I agree that will go in circles, but not forgetting that they're both needed in a well rounded analysis.
We can guess with some ballpark accuracy what is possible in 2019, 2020, 2021 or whatever. But picking actual years which is what the last few pages seems to do is far more complicated than silicon and power. Just pointing that out.
Of course, all aspects of the question remain fluid. I guess we have been doing both for quite a while, first picking a year as an exercise (testing rumors of the day) to figure out what would be possible, and rinse, repeat, for another year. Once we figured out roughly what can be done in 2019, 2020, 2021, we can then argue which one is sufficient to make a business case for a next gen at that time.
Based on past generation transitions, next generation timing is somewhat tied to the ability to produce a significant improvement at a maimstream console price point. If there is no significant technological improvement, the current generation will continue to be milked until something happens. Big jumps are mostly tied to node transitions, new cpu/gpu architectures, new paradigms (from sprites to 3d, gpgpu, new distribution models or media, etc), or all that together.
2019 is very unlikely. 4.2/6TF is not going to look much worse than 8-10TF. That makes the case for waiting a couple more years. But OTOH, the cpu could be mid-gen's achille's heel. So a much better cpu, paired with only a 10TF gpu, would be a good argument for a next gen transition, then gpu upgraded to 24TF or something for a later mid-gen. New hardware with ray tracing helpers could be magical for (hybrid) GI. I think it's not crazy, just unlikely.
At the other extreme we have a 2022 launch on 5nm+ or something. Waiting too long is dangerous, sales dip when the generation lingers, and competitors with fresh new hardware get the upper hand. Unless there is a major tech ready only in 2022, it creates an up hill battle.
I see 2020 and 2021 as good candidates as long as they get the significant tech improvement on time. So my biggest question is... Will they?