Well, no, Wallace was pretty terrible.
There's a difference between overpaid and terrible. Wallace isn't so bad that he should be out of the league. He is still a player worth more than the minimum who can at least be a useful backup, especially if he can continue to defend multiple positions.
MySynergySports gives him 0.84 PPP on defense, ranking 119th. For comparison, Jeff Green gave up 0.81 PPP, ranking 71st, while Paul Pierce's 0.79 PPP was good for 57th.
I think Wallace is in decline, but it's not clear how much of his cratering next season was due to injury and loss of confidence and whether he can recover at least part of what he lost if healthy and in a new situation with a defined role. Given how the Celtics have been hit hard with injuries in recent years, perhaps Boston isn't the best place for him to recover.
He's probably not going to be a guy who is going to recover enough ability to make a team want him at $10m/year, but he might recover enough to be a guy that some team would rather overpay than the guy that they signed for $7-8m per year.
I see some of the deals being sign this year resulting in some buyers' remorse and I could see Wallace being moved next summer in a trade with a swapping of bad contracts similar to the 2011 Bucks-Bobcats-Kings deal that involved John Salmons, Stephen Jackson, and Corey Maggette. A poorly-run team may even give the Celtics a draft pick for the honor of overpaying Gerald Wallace. After all, the Kings gave up Beno Udrih and moved down in the draft so that they could take on John Salmons' contract.
The Nets are good. The Clippers are good. The Maloofs don't own the Kings anymore. Which team is Danny supposed to get to do something stupid?