I think we agree Pogacar would and will be able to win also at least one Giro and one Vuelta. The big question is, what about the Monuments he hasn‘t won yet.
I am sure he could win San Remo, just like Nibali did (San Remo, however, is the easiest to finish and hardest to win, as they say). An attack on Poggio or even Cipressa would be needed, and VDP, WVA and Ala should not have their best of days then. So here, Pogacar needs top spring shape, a good day and luck.
The cobbles remain a questionmark - we just don‘t know how Tadej would handle them. In general, he seems to have very good bike handling skills, and he‘s not too light (not <60/65 kgs).
I think RVV is a race he could win. He should be good on these climbs. Valverde did RVV once, and immediately finished 8th, which is really impressive.
For Roubaix and Tadej, there maybe remain doubts. Normally also here, he should have the engine that‘s needed.
My personal guess is he won‘t do RVV and Roubaix the next years. The Ardennes triple of AGR, FW and LBL looks better suited to him. In Roubaix, especially, danger of injury is always a factor. The Ardennes races with their good roads are safer, to stay healthy before May‘s altitude camp and Dauphine and Tour afterwards.