So a couple thoughts on the proposed deal:
1) I don't buy at all that the Bucks would be looking to keep Jaylen. I get they don't have their picks moving forward, but they were below .500 last year even when Giannis actually played. They aren't going to magically be better subbing in a worse player in the Giannis minutes, and without their picks they have no real way of improving. Meanwhile Jaylen's trade value will never be higher than it is right now coming off a healthy all-NBA season. They have to trade him.
2) If I was the Bucks I'd be trying to move Turner into the C's TPE. Giannis/Brown is almost even moneywise straight up, but the C's are one of the few teams in the league with an exception big enough to just TAKE Turner without sending back money. Turner still has 3/84 left on his deal and while he's a good player that's clearly an overpay. Meanwhile Kuzma is a flat expiring so I'm not really sure why it's so important to off load him. Maybe the Bucks think they can move Turner for value elsewhere, but can they do that without taking back bad money?
3) What else is in the deal? It looks like it's not going to be Giannis/Brown straight up, Celtics will send other stuff. But what? Hugo is an obvious candidate, as is the #27 pick. The Celtics could do any combo of those two plus 27', 31', 33' 1st rounder (as long as they wait until after the draft to make it official). They could also take back Turner/Portis/Kuzma as mentioned. Or include other young guys. A lot of ways to add value. Personally, I'd draw the line at Jaylen/Hugo/#27, especially if we have to take back Kuzma. Save our future firsts to fill out roster holes.
I have mixed feelings about this trade. Jaylen is my favorite current Celtic so it would be hard to see him go, but Giannis is probably my favorite non-Celtic. I do worry a lot about his calf problems for a 6'11 guy entering his age 32 season. I guess this is a trade you have to do, but i don't think its the slam dunk obvious choice some are making it out to be. There are ways it could backfire big time.