Robson would have been regarded in a wholly different light by the masses if he had gone off to Italy and competed in the most respected and regarded league of all-time. His England career was ruined by injuries and his legacy as a United great is, of course fantastic and amazing for us, but in the greater scheme, he is not the household legend he would have been across the world because he chose to stay and perform in sides that were far beneath him. Bryan Robson was the equal of any 2-way midfielder during his pomp, but the amount of people who will automatically place him in that *top* tier is far fewer than it should be.
One could also say that the way he was living (heavy drinking culture), there was no way he'd even consider going to an almost tee-total, clean living footballing environment. He's a United and British legend, but his world-regarded legacy suffers for his decision to carry United.
De Gea has had a career that splits wildly at the point he got that insane deal from us. At his peak, we were letting him down as the form he was displaying had many here even comparing him to Schmeichel, but in his decline to what he has been for the past few years, he has been paid in accordance to a level he's not close to anymore - he earned the plaudits and pay hikes and then things went south, but ultimately, he, in personal terms, has at least made a fortune via optimal earnings he could not get elsewhere. With the dichotomy of his career, he's harder to call than players who only decline via age or injury (see Robson, or indeed Ruud).
I don't think Ruud's situation is the same as the other two you have put forward - he was never downright better than the teams he was in. He was our star striker in a team full of star players in other positions. There's no point in time where he was head and shoulders our best player like the other two.