I agree that the trade was sensible, given Irving's age and what he's already accomplished thus far in his career. (Also, the certainty of knowing you've got Hayward in the fold, which you didn't have on draft night.)
With that said, this was not the Celtics' "only" option. Another option would have been simply to keep the roster intact this year, throw out a team of IT/Hayward/Horford/etc, and then negotiate with IT in the offseason. The worst case scenario would have been for IT to walk, in which case you're going into the 18-19 season with Marcus Smart (assuming he'd be re-signed) and Terry Rozier as your PGs, which is a step down but not a disaster. Of course, maybe the market for IT would have proved soft, which is what a lot of people believed would be the case even before the latest news on his hip, and you could still resign both him and Smart.
In that case the Celtics would have stayed competitive but still had a boatload of assets -- Nets pick, Lakers/Sac pick, Memphis pick, etc. -- to throw at Anthony Davis or whoever else when the time is right. That would have been a fine alternative, I just think I like the Irving scenario better.