Bradford is indeed a solid QB when healthy and behind a solid Oline.
I don’t think the Cards Oline is solid, so I see Rosen getting a look somewhere between week 8 and week 12.
I think it may come even earlier if they start out 2-4 or something like that.
Now that said, if Bradford stays healthy (big if) and they are losing they may decide to let him sit and learn all season since other than Fitzgerald and maybe if Johnson plays like he did in 2016 they have shit on that roster for a young QB to work with.
And your point about the OL is a good one. Facing defenses that will be coming after the rookie QB with a bad OL is not really a good strategy.
Personally I hope he and Garappolo completely flame out. In the last several years SEA and AZ have elevated themselves to the level of 49er dislike because of their smarmy fucking asshole HC's. I want nothing more in the NFL for those two teams to suck while the Rams beat them each twice a year on their way to the postseason.
I'm telling you my favorite game in many years was the last game against the Seahawks. Maybe since the SB or the game against the Chargers in 2000.