If Golden State is serious about contending right now, they should do the Pierce for Barnes, Richard Jefferson and injured Brandon Rush right now.
They get Pierce for this year and next for 90% of what RJ and Rush will be making (and there's no guarantee Rush will be healthy to start the season). they have a second round pick on a tiny salary to back up Pierce, in Draymond Green. They are giving up a high draftee with a lot of promise, but you're not going to go far in the playoffs with a ton of young guys, and they have Klay Thompson to play the young un wing role.
I like this trade for the Cs. They get a young Pierce replacement with a bucketload of talent, in Barnes, a veteran who will have a big expiring contract next year, in RJ, and Rush, who will be useful when healthy next season. Pierce gets to go back to California and will have a real chance to contend out there, assuming Bogut can get on the court.
I also like the Clippers package of Bledsoe and Butler, along with Chauncey, but I don't think the Clips are going to make that big a move. For Pierce, this landing spot would probably be the most palatable.
Trading Pierce to the Bucks would be unfair to him, although I'd probably do it if I were the Celtics' GM.
Regarding the skepticism that teams would give up young prospects for an aging vet, most teams won't, but one has to look at each situation differently. Then Clips might do it because they think they can win the title now, and that they're not going to sign Bledsoe to a big extension down the road anyway. I'm guessing future UFA Chris Paul would appreciate ownership taking on Pierce to help the team get over the top. The Clips will need more half court offense to win playoff series against top teams, and Pierce can give them it.
The Warriors have to decide whether they want to run with the big boys now, or be a contender in two or three years. A lot can happen in the next couple of years. I'd take the shot now if I'm them.