Why not? If the Celtics aren't willing to give up a second-year player or a rookie who hasn't played a minute of NBA basketball, they aren't getting Kyrie. You can play hardball when you have the leverage. The Cavs may have been rushing into a trade, but that's their own fault. They had Irving—one of the eight best players in the league—under contract for two more years. Worst case, you hold onto him. Ainge wasn't going to stumble into one of the seven guys better than Irving because those guys don't get traded. The Celtics may have been bidding against themselves in the end, but Irving isn't going to damage his reputation by playing like [Mark May] just to spite the Cavs. Keep him if you don't get the Godfather offer.
I'm not sure how you think the Cavs are better. You could hide Irving on defense. Thomas is one of the worst defenders in the league. And Crowder hasn't lived up to his defensive reputation in years. We basically made this trade for Thomas on a one-year rental and the Brooklyn pick. They could've done better, and if they couldn't have, they should've held onto Irving.
I think we should all remember how overvalued the #1 pick is, too.
Look at the last 10 years of #1 overall picks. How many of those players are special? Towns and Davis for sure. Rose, Irving, Wall, and Griffin all had the potential to be the second-best guy on a title team. Bennett was terrible, Oden got hurt, and we don't know what to make of Simmons or Wiggins yet. And that's if we get the #1 overall pick. The chances of getting a star decrease considerably if the pick falls to #2 or #3. I'm not saying we shouldn't have wanted the Brooklyn pick, but it isn't a guaranteed home run. Picks like this are always sexier on paper. We're all enticed because we may wind up with the next LeBron. But there's just as good of a chance that we wind up with the next Kwame Brown.