My take:
It's a good trade for the Celts and a great trade for the Cavs.
For the Celts: Getting off IT without committing to a huge contract, freeing up minutes for Hayward/Jaylen/Tatum and getting a massively popular (insane shoe sales / Uncle Drew marketing) superstar in his prime for 2 seasons+ is a big deal. Kyrie is 3 years younger than Jimmy Butler and 2 years younger than Paul George. He is just entering his prime now (He's 25, 27 typically is NBA peak). You always make the trade for the in prime star if you don't cripple yourself in the process. Always.
It's a steep price, especially the Nets pick, even if the Nets might be on an upswing now after acquiring Russell, and Jae's very friendly contract. Zizic was also an appealing prospect, but the salaries had to work. When you acquire this many picks, however, you can't keep them all (even with foreign stashing). At some point you either have to let veterans go in Free Agency for no value or trade multiple players to marginally upgrade a starting role. You can afford to overpay a bit in this scenario, as marginal upgrades at the highest level are both more expensive AND more important.
Also, if you think about it, the great contracts that Jae and IT were on already served their purpose for Boston. They allowed the Celtics to sign Hayward without having to trade assets for him and giving up only minimal cap casualties (had to trade Bradley at a bit of a value loss, but not egregiously so considering his contract status). Now that Boston was over the cap, the contracts would have more value elsewhere as trade chips, and they were successfully used as such in this trade.
Considering how popular Stevens is, the respect the front office has around the league and comments Kyrie/Ainge has made, I would be surprised if Kyrie leaves after 2 years. He'll opt out, but they'll pay him. Especially if some of the other talent like Tatum/Brown pan out. They've set themselves up as a top 3 seed in the East for the next 7 or so years. Whereas IT and Crowder guaranteed nothing in the long term. Draft picks are coveted because they give you the best chance to get a star, but they're especially risky... unless you can trade them for a bonafide star, which you should essentially always do if the price is reasonable.
For the Cavs: This is a masterful deal for the Cavs, especially relative to what was being thrown about in rumors. It accomplishes both short term keeping the team competitive with Lebron here, and reloading the roster if he decides to leave next year with the high pick. IT even expires same time LeBron expects to leave, so they won't be cap stuck. They also saved a boatload of moeny this year, something like $15 million, thanks to the very friendly deals that IT and Crowder were on. They got exactly what they wanted, even with limited leverage.
The leverage aspect of this trade is the reason why it isn't a slam dunk for the Celtics. Could they have traded a worse pick in place of Brooklyn's if they waited? I guess we won't know, but the return is still good as is.
