Cerny mentioned around the launch of Pro in 2016 that he felt at least 8TF would be needed for guaranteed full native 4k. Can't help but wonder if he was privy to info about where Navi was likely to land in terms of TF calculation. It would also partially explain why Pro only hit 4.2TF. If your next gen console is 'only' going to hit 8TF-10TF, then why try and launch a monster of a console and make the next gen leap look much less impressive?
This would make it tougher for MS to market Project Scarlett since they will either have to really push the GPU performance and pay a hefty price for that, or possibly sacrifice CPU cores to keep the die size and cost down, or use two separate dies, still increasing costs. PS5 could be 8.4TF and it will seem like a much larger and more worthwhile leap in comparison to a 9TF-10TF Scarlett. Even if PS decided to shoot for 10TF and that happened to be where Scarlett landed as well, then it would still look better for PS than MS, due to the difference in performance gains on paper. PS will be able to use this to their advantage, where it would hurt MS, making their advancement seem weaker.