I'm not a Dez "homer" (or a Dak "homer", either), but I believe the responsibility lies with both of them. Dak's best throws are short to intermediate routes. He does not throw a good deep ball or deep out. The throws he happens to be weakest at just happen to be where Dez excels. Slant routes are just about the only strengths they have in common. They're not a good fit for each other. Out Dez on a team with a QB whose strengths aligned with his, and I believe we'd see a highly productive receiver again.
Dez is who he is, however. Dak will improve on those weaknesses through experience, but I don't think Dez has time to wait on him. Perhaps it is better for all concerned for Dez to move along after this season.
I hope I'm wrong about that, however.