Cooper is what you really want in a #1. Size, speed, route running, has the QBs confidence.
I won’t say Dez is a #2 because he was way too talented, but Dez was at the point for a few years where he couldn’t pull away from the elite corners, and he couldn’t run routes well enough to separate from them. That’s NOT what a young QB needs in a #1. Even Romo seemed to develop better connections with other receivers over the years and his TE, then going to Dez for huge plays.
Cooper has already shown that even a top CB cannot take him out of the game. He will demand double coverage. That would have then made Dez a monster against single coverage by a lesser CB.