1) He's not early prime. For a player who, to date, has relied heavily on athleticism, he's near the end of it.
2) The one thing Spurs fans have never gotten with Bonner - it's not always about him getting & making open looks. His mere presence on the floor forces the opponent to stretch out their D outside of the paint. Since Nene can't hit jumpers anymore, they don't have a big on their roster than defenses have to respect outside of the lane. Driving lanes for Wall have been tough to come by.
3) "The Spurs have attempted to trade Bonner before but to no avail." Not often, and they've never shown a willingness to just give him away. Just because the pieces didn't work doesn't mean that's not a movable deal.
4) For all the talk of Blair being a ticking time bomb, he's 24 and hasn't suffered a major injury in over 7 years. He's got a career PER of 17 & has adapted aspects of his game (i.e. floater) to play against bigger opponents. And his game may play better in East with the plethora of small ball teams in that conference. I don't agree with the Wizards, but there are reasons for them to like him as a bench guy in a low risk/bigger reward type move.
5) How movable is Ariza? He's paid beyond his production. His best value is as a role guy to a contender, but how many contenders have a need, the roster space, and the cap room to nab him? His expiring contract only has value to teams that are looking to shed longer term salary, which I'm guessing the Wizards don't want to take on.
That's not to say Washington would do it, but they might. Move a big salary that they have no use for, gain a stretch 4 with no long term commitment + sign an energy 4 to a low risk deal.