It will run @60 fps on PS5 as well. It was the CPU that held back the current gen. Too early in the gen for that to happen in open world games.
At FullHD yes, but at 4K games are 100% GPU-bound, if you put billion of polygons, tons of effects, RT and what's not, it won't matter if those 8C/16T Zen2 CPUs are being utilized only in 20%, if the render time if greater than 16ms then you just won't see 60FPS. 1440p seems to be the perfect resolution that balances the CPU and GPU usage pretty equally.
As for the second part of your sentence, I don't know, just look at what happened during this generation - most if not all launch/early titles ran at native resolution, it was a bit later when dynamic resolution started being used, then reconstruction kicked in, and in the end both technologies were used simultaneously, that's the beauty of simple, PC-like x86 architecture, all the power if easily accessible for the devs from day one, that's why launch titles like TO1886, Ryse, Driveclub etc. still look so damn good, if not even better than many games from even this year, but the drawback is that the devs can't squeeze anything more from those consoles than what's on the paper specs, they can keep pushing the visuals but that has to come at the cost of resolution, and I have very little doubt we will see the same trend happening yet again on PS5/XB1, where the launch/early titles will indeed run at native 4K, which in 2-3 years time will be replaced by dynamic or reconstructed resolution, or the mix of both.