I really don't know which nba forwards guys are watching to be so down in TT. Theis was not very good this year. He has played much better the last month, close two months than Theis did this year. Neither get starter money. Kanter, according to SOSH, was shit . Somehow he is shooting. 60% averaging a double double for a better team than the Celtics. Whoever replaced TT in your scenario, unless he's an all star will quickly be SOSH's whipping boy.
Perhaps our lovely guys with the ball are not good at using bigs. We are all emotionally invested in Tatum and Brown, we blame others for bad plays involving them.
As a big I would hesitate to sign here. Tatum, Brown are bad at hitting the roller, they tend to take an extra beat or 6 to see which shot they have before looking at the roller. Flipping around I saw Theis slip a screen get a quick pass for a dunk and remembered he basically quit trying that here. It took perhaps the GOAT of lob catchers getting minutes until the roll game was used at all.
I think you're conclusions about Brown and Tatum's current passing are fair, but everything else here ignores the facts. First of all, the Celtics have attracted plenty of free agents lately. Horford, Kemba, Hayward... I don't think there's a single NBA center with talent who wouldn't want to play with these guys.
You're also ignoring the fact that Tatum and Brown have improved and will likely continue to improve as passers. Both of them are young and have made huge strides. Tatum is way ahead of Brown, and flashes excellent passing. I think next year he averages over 5 assists and wouldn't be surprised to see him settle in around 6 or 7 as he gets into his prime.
As for Kanter and TT - neither of them are great in the pick and roll, and Kanter is better than TT. TT got mediocre hands, needs a perfect lob, and gets tunnel vision under the basket. Theis was a better roller, Horford is probably still better, TL is obviously the best of the bunch. Kornet, for all the shit he gets on this board, is better. Kanter was not lamented for his offense, anyway. He's a bad defender and he'll get hunted in the playoffs, just as he always has. And Portland will see another quick exit, if I had to guess - why? Their terrible defense, anchored by Kanter.
And TT is not playing forward, he's playing center, and he does only one or two things well as a center: switch and rebound. He rarely makes mistakes on defense, but almost everyone can score over him. He's not an NBA starting center anymore, not on a good team. He just doesn't provide the rim protection you need to anchor a great defense or the vertical spacing that would open up the offense. I like him fine, but I'd almost prefer to see Kornet out there. At least he can get a block.