If you could have any qb in the league as they are right now, it;s a short list of guys you would take over Stafford. That means he's better than all but a short list of qbs, the difference being, none of the other short list qbs was going to be available.
He's not one of the twilight years FAs like Manning with Denver or Brady with Tampa. It's just a rare set of circumstances, like what brought Brees to New Orleans or Palmer to Arizona.
So is an offense better if you add an experienced top veteran qb who still has top arm talent and is still relatively young?
That was a rhetorical question.
Plus though this is murkier and speculative, another advantage is that as I see it, McVay is in a honeymoon period with Stafford and so will listen to him and build around what he knows he can do. There's that and the fact that Stafford is a fit already in a lot of respects--he is said to be very good at play action, for example.