Yeah, I definitely find it odd that Ainge was completely unwilling to spend any assets whatsoever on a proven superstar like Paul George or Jimmy Butler but more than willing to throw away a bunch of key pieces (the Brooklyn pick, a PG who was getting MVP votes last season, his super roleplayer starting SF, a good solid rookie big) for a guy who in his last stint as alpha dog was routinely getting outproduced by IT.
One can make the legitimate argument that a Horford/Morris/Brown/Hayward/Kyrie lineup wouldn't be able to beat their Horford/Amir/Jae/Bradley/IT squad of last season.
I think the main point is, that Stevens doesn't need to change his system at all with the addition of Irving for IT. The Celtics are used to play off a scoring PG and they are used to cover that PG on D. Hayward is a good defender and an overall upgrade over Crowder. Smart is able to take Bradleys role on D, while Tatum and Brown mature. Amir wasn't a heavy minutes guy, because Stevens likes to downsize. Morris fits Stevens prefered playstyle way better and could serve as a full time starter at PF.
Irvings last stint as an alpha dog was 2013-14 and Irving was 21 years old, playing on a team featuring young and stupid Dion Waiters and Luol Deng as the next two best players. I think it's reasonable to expect a bit more out of him after plenty of finals experiences and with a much better team tailormade for his playstyle. The biggest questionmark with Irving is his health, but IT's hip made this red flag a lot less scary.
Irving is pretty much the same player IT's is, but younger, bigger, currently healthy and under contract for the next two seasons at a reasonable 20 millions per year. IT won't take anything less than the max next year, if he gets healthy and he will be past his prime once Tatum and Brown are ready.
Jimmy Butler feels a bit too ISO heavy at the wing for the ball moving addict Stevens. I think Hayward is the better fit (better shooter/ball handler).
Passing on Paul George is the real questionmark, but maybe his affinity for the Lakers played a part in that.
I think the trade will turn out ok for the Celtics. Yes they overpaid a bit, when you look at the return for other stars like Butler, but Irving is a solid fit and a top-5 PG in his early prime. I'm a bit worried about the chemistry and the team identity, because Bradley and Crowder seemed to be key players in that regard, but Hayward looks like a guy from the same blue collar mold, so maybe with him and Smart as the most likely leaders it will be the same "no nonsense" type of approach that made the Celtics an enjoyable team to watch.