If IT is healthy and performing close to his previously level Cleveland is pretty dramatically improved. They have added Crowder who all the stats guys call a top 25 player in the league. Even if you think that is overrated he fills a critical role player slot for them where they had nothing previously. The have also added Derek Rose, who isn't the player he was before, but can be used more sparingly there, and can be an outstanding bench player.I agree with some of your point but it is only true if Thomas is truly healthy and playing at the same level he did this past year. If IT is healthy and you add Crowder to the role players they are just as good as they were last year if not better. The only thing in my mind that could keep that from happening is if IT is not as impactful because he doesn't have the ball in his hands all the time. But, he's shown in Boston that he can play off the ball and he'll need to with LeBron dominating the ball when they are on the floor together. However, they don't have many young guys whose improvement over an 82 game season could make the team better in the postseason. They might not need that to happen because their top 6 may be good enough to get them out of the East. However, I can't see Gilbert trading the Nets pick away for a 1-year shot at a title. They made the trade with Boston so they could start a rebuild if LeBron ends up leaving. I highly doubt LeBron would tell Gilbert he'd stay until after the season anyways so there's no way they trade the pick before knowing LBJ is definitely staying.
Boston, on the other hand, can be much improved from where they left off last year. Let's start with the known quantities. We know that Hayward is a much better 2nd option than they had last year and Marcus Morris should be an improvement over whoever started next to Horford. A top 3 of Irving/Hayward/Horford is better than they had last year and Morris is going to be better than Amir Johnson. Where the jump for this team could really come is how much of a leap young players like Rozier, Smart and Brown take and how NBA ready Jayson Tatum really is. We know how much the team relied on a deep bench last year and while the starting lineup is going to be better they still need the players behind their big 3 to perform at a high level in order to have a shot at the East. I hope they add some rebounding help like Thomas Robinson as it is sorely needed.
Boston adds Heyward, and they lose Crowder. They Add Morris and lose Johnson They also lose Avery Bradley.
Boston may and probably will be better than last year, but have they improved much more than Cleveland has.
And most importantly. Last year's conference finals wasn't even a tiny bit competitive. The Celtics won the game where Cleveland basically fell apart and blew a 21 point 3rd quarter lead. The other 4 games weren't even close.
So lets say the Celtics have improved more than Cleveland this year, has it been enough to close the gap in 2 teams that were badly mismatched in the spring