I am not attached to Thomas and would have played out the season and then let him shop for deals. I think I've been pretty clear on this in many threads. So please don't say my opinion is colored by overrating Thomas, except to say that objectively I just don't see the Thomas-Irving gap that is worth the price Danny just paid. And yes that includes having Marcus Smart, starting PG, as the plan for 2018-19.
To me, there are several arguments you can make, and I don't find any of them compelling.
Entertainment wise - I think Thomas is the easier player to root for, has more of an underdog thing going because of his height. Trade that for Irving's incredible handle and finish which is fun to watch. Regardless in terms of just fan value watching the team, I do think coming into this year this is at best a wash.
Value on the court - Thomas and Irving are very similar, achieving most of their value on the offensive end. You can argue that Irving has more ceiling simply because he doesn't have Thomas's physical disadvantage. That is fine, but he will also need to take advantage of that far more often. Essentially, though, Irving has never done anything to say that he's going to make the team better on his own, and in this way, I'd be worried about giving him a max in two years and ending up with a post-Horford/Hayward team with Irving as the centerpiece. A lot can happen between now and then but a lot could have happened without this trade being made.
Value Part 2 - Stevens system - Thomas even though he was by far the Celtics best player offensively was not a ball stopper. Irving despite playing with one of the best players in the history of the NBA often was. You can argue, again, the Irving has the skills necessary to be coached into ball movement and playing the "Celtics way." But again, this is what I'm talking about with the magic wand.
Value Part 3 - playoffs. Here's where Irving has historical advantage. Always, though, under the shadow of a much better player. Shade he will not enjoy in Boston. As well, once you gave Thomas a couple of options, he was able to light up a few playoff games himself.
Window-wise - Irving is a very good player. While I don't see the gap as wide as some he is better and more versatile than Thomas. But, is he better enough? Is he a rare enough player that you have to make this kind of deal? Will the Celtics even be better than the Cavs during Irving's tenure as a Celtic? Everyone seems to be making the assumption that LeBron is going to cut and run and they will deal Love for whatever they can get and rebuild again. What about the situation where LeBron convinces a star to come to Cleveland and now that BKN pick gives them the trade piece they need to make it happen? Or simply if LeBron doesn't leave at all? Even beyond that, can they compete with the iron of the West with this team?
If it weren't for Stevens I would be much more emotional. To me on paper this is a completely shit trade in terms of present day and in terms of future planning. It doesn't get you closer enough to immediate goals at the price of longer term goals and continuity. The only thing it achieves is keeping you from being the bad guy who didn't re-sign Thomas when you had the chance. I hope Danny's not that much of a coward. (I don't think he is, either). So I have to assume they know what they can do with Irving. But right now I still don't like it.