I am only stating that if the R6 II comes out before R1, I would not expect a stacked sensor in it because it would take away a key selling point for Canon's flagship.
Stacked sensors will eventually trickle down to R5, R6, and beyond, just don't see it happening this quickly in the R6 II.
One thing to consider is Canon does not just have to compete with its line up, but with the competitions line up.
Even if they put a modified R3 sensor in the R6ii, it still will have firmware handicaps in order to keep the R3 a more "professional featured" body with more advanced bell and whistles. Like a faster shutter speed, buffer speed, 4k 120, CF Express, etc.
The truth of the matter is that Sony released a response to the R6 with the A7iv. The only real edge the R6 has over the Sony is the minimal 4k 60 crop, otherwise in my opinion the Sony has an edge on most of the R6 features.
Sony is releasing the A7Rv in 2 weeks. Since the R5 is 2 years old, it would be safe to assume that its rumored new processor and 8k would give it an edge up on the R5.
Canon has been sitting in second place and Sony is ready to release updated bodies.
It would be safe to assume a Sony A9iii would be next and will probably one up the year old R3.
Once the forever rumored R1 is released, a Sony A1ii will be coming out right after to outmatch it.
Canon either has to take an aggressive approach and maybe take a margin hit to bring features like a fast readout stacked sensor to their lower tiered bodies before the competition does in order to keep the ecosystem alive and not lose more people switching to Sony. Especially when Sony has lenses.....
Now a consumer would have the choice of a stacked sensor R6ii over the A7iv until the next A7v releases in a few years.
Or they wait for an A7v first and then release an R6 to one up it. But by then the other bodies in Sony's line up would have out shined Canon's.
Hence why I think Canon will finally go on the attack and try to push new product out faster and first.