I really think the Celtics are better than the Cavs next season. IT isn't that good. He's certainly (imo) not as good as Kyrie when both are healthy. Kyrie has his flaws, but LeBron is going to miss Kyrie's elite level of getting buckets in tight post-season games.
I really think that the roster they have now assembled, along with having elite level coaching, that the Celtics are ahead of the Cavs. IT isn't as good as Kyrie. This deal is good for the Cavs long-term but I don't think it makes them any better next season. The Celtics added Kyrie Irving, Gordon Hayward, Jayson Tatum, Marcus Morris and Jaylen Brown should be improved enough to contribute next season.
The Celtics added a lot. The Cavs added nothing and swapped Kyrie Irving for IT.
The Cavaliers will have probably the #1 picks back to back seasons, but they will still be completely starting from scratch. That roster is about to completely implode. IT and LeBron are both leaving. Dan Gilbert is not well liked, and Cleveland isn't a desirable place to play. We will see what happens to Cleveland even with almost guaranteed back to back to 3 picks. It should be noted that before LeBron James they were shit, and after LeBron James they were shit. The only reason they won is because he returned- and he's going to leave again.
You're ignoring the single biggest thing in the trade for Cleveland which was getting Crowder. Crowder is a top 75 player in the league and one of the few true 3 and D players in the league that can check 2-4.
Secondly, Irving is better than Thomas, but it's not the difference you are making it to be. They're both legitimate All Stars. It's not like Thomas was scoring his points inefficiently.
Per 36
Irving 26 per game on 47% and 40% from 3 with 6.0 assists
Thomas 31 per game on 46% and 38% from 3 with 6.3 assists
Advanced
Irving usage 30.8
23.0 PER
.170 WS/48
2.5 BPM
Thomas usage 34.0
26.5 PER
.234 WS/48
5.4 BPM
Playoffs
Irving 26 and 5 on 47% shooting
Thomas 24 and 7 on 43% shooting
Both are awful defensively, Thomas is just probably the worst in the league instead of being like Irving as one of the worst.
Point being, Celtics upgraded, but it's not some sort of huge leap for the 2017-2018 season.
Lastly, for those touting Tatum. I could be wrong, but I find it hard to believe he will be much better than Jabari or Ingram were in their rookie seasons. Both of which were average players at best. Tatum posted some great numbers in the summer league, but a lot it was done in isolation on some high degree of difficulty shots. Shots he isn't going to be getting for the Celtics as a young player with Hayward and Irving on the roster.
Have the Celtics shrunk the gap between them and Cleveland, absolutely. As long as LeBron doesn't pull a 2010 playoffs where he's on his way out, then the Cavs are still a drastically better team than Boston in 2018.