I don't think Dion is that overpaid, to be honest.
Dion & Johnson are in the same boat, they're #1 d-men by default. They're not real #1s, they're above average #2s. If you look at all the good teams in the league, Chicago, LA, Anaheim, Tampa, Pittsburgh, Montreal, Minnesota, New York Rangers... They would both be #2s on those teams and they would be slotted perfectly. Have players like Keith, Subban, Hedman, etc do the heavy lifting while having Phaneuf/Johnson in a supporting role. Same thing happened with Petry. Was Edmonton's #1 d-man by default, looked terrible on a lot of nights, was even scratched a few times, then he gets traded to Montreal, gets slotted properly as a #3 and he looks ten times better.
But like Petry, they're not on those teams and they're the top d-men on bottom feeding teams. It's not a coincidence.