Ariza being overpaid has everything to do with his trade value below. I can't even believe you even tried to suggest it doesn't. He doesn't have a lot of value because he's not a worth his contract. If Ariza were making $2 Million a year, no one would care that he's been pretty bad since leaving the Lakers. But since he's making so much money, he's not as attractive to teams. That's simple trade economics.
Your comparison of Ariza's and Garnett's situations was just silly. Garnett was traded twice over the course of six years because he asked/allowed himself to be. Ariza was moved twice on the same contract (three times total) because he teams didn't want him anymore. Houston moved him because they knew he wasn't worth is deal and thought that Courtney Lee could do what Ariza did. Washington took his contract so they could get Okafor. The Hornets only got a second-rounder out of that deal, so you better believe they considered getting rid of Ariza an asset on the trade ledger. Neither time did Ariza's former team get back a quality player for him. I don't know why you believe Washington will.
You can polish him up as much as you want to make it seem like he's some great asset, but the fact remains that he's been pretty bad since leaving a well-structured offense. He would be a good fit for the Spurs, but he's not worth much to any team right now. It says a lot about a team's opinion of a player that they not only drafted his replacement but also handed out the full MLE for a player to back up his replacement. Even if the Wizards have as skewed of a view of Ariza's value as you're demonstrating, they just completely undercut his value this off-season. No way they get anything like you're suggesting they will. If they want Blair and think he can help them win games, there's no reason to keep holding out.