I'm not sure you understand how difficult it is to put the ball where the receiver wants it to be. Most quarterbacks deliver to a catch radius because it's pretty much impossible to place a ball perfectly on point every time.
It's easy to watch Dez's play with Dak and see that Prescott either wasn't comfortable or wasn't able at that point in his career to place the ball where it needed to be with Dez. With receivers who don't really create much separation, it takes more accuracy to put that ball up over the corner where the receiver can win it. Too low and the corner's body is in the way. Too high and the receiver can't climb the ladder enough. Too wide and the receiver can't adjust against the direction his momentum is taking him.
I'm not discrediting the catches; those were Dez's strong point. If the QB put it up there in Dez's zone, he was going to go get it. Again, you can go back and listen to what Romo had to say about where the ball needed to be placed.
Now, I don't want to put the blame on Dez's falloff squarely on Prescott. Injuries also took some of Bryant's ability to win the jump balls as well as any explosiveness that he had. He essentially became a one-trick pony and he wasn't near as good at that trick as he had been. (This is when the route-running complaints emerged, which were legitimate mainly because Dez never really had had to focus on route precision before.)