well it isn't just 2 years of Sexton. It is whomever Ainge drafted or traded for, Zizic, Crowder, and Thomas. It is 2 years better growth from Rozier, Brown, and Tatum. It is 2 years without the headache of Irving.
When you think of it like that, I think the trade isn't made, especially when you see the lack of success Boston had with Irving. If he had led the team on some deep playoff runs, then sure, you make the trade, but he didn't, and then he left Boston with nothing.