Again, I think you're undervaluing defense. Thompson is an ELITE rebounder (in terms of % gathered) and
basically does EVERYTHING except shoot jump shots. He's a better version of Hassan Whiteside because he's actually a better defender than he looks (rather than worse) and Whiteside is going to get a ridiculous contract. Why is he a "role player"? Is CJ a "role player"? Because he has more holes in his game than Thompson does.
Also, what's with this "3rd reliable scorer" business? If your first two options are struggling it's because the D is loading up on them. If that isn't giving one of the other three players open shots then you need a new coach more than anything else. The "3rd reliable scorer" is THE REST OF THE FUCKING TEAM.
He's not an elite rebounder. His career TRB% is 17.1. That's above average but not close to elite. You need to be up around TRB = 20 to be considered an elite rebounder. Hassan Whiteside has a career TRB% of 23.7. That's elite. Even Ed Davis is slightly better than TT for his career at TRB% = 17.3, but over the last two seasons, playing the most minutes of his NBA career, he's been at 20.0 and 18.7. That's much closer to elite rebounding that anything TT has done. And, Davis is here, wants to be here and is on a much cheaper contract.
There are multiple reason's we need a 3rd
reliable scorer.
First, we need someone to pick up the slack on nights when one of Dame or C.J. are having a bad shooting night. That's not always the result of the other team playing great defense. We've all seen what a streaky shooter Dame can be (some would say feast or famine). When one of your top 2 is having an off night, a reliable 3rd scorer can keep you in the game.
Second, a
reliable 3rd scorer would make it more difficult for teams to lock down Dame and C.J. It would change the way other teams game plan for us, especially in the playoffs. Without a reliable 3rd option, teams game plan to camp down on Dame and C.J. and hope nobody else beats them. Aminu comes up huge about once per series, but that's because other teams leave him wide fucking open. Having a reliable 3rd scorer makes it harder for teams to focus their game plan on just stopping Dame and C.J. That makes life easier for them and I'd rather have them have to work less hard for their shots than depend on Aminu to get hot and knock down some open shots once every 5 games.
Third, a
reliable 3rd scorer is insurance against injury. Injuries are part of the game. They happen to all teams eventually. Having a proven 3rd scorer, who can step up and get you 20 points a night when needed is something every championship caliber teams needs. Without it and you lose one key player at the wrong time and your season is over. Having that 3rd scorer helps weather the injuries without sinking your season waiting for someone to get healthy.
So ideally, I'd prefer adding a player that gives us both improved defense and a reliable 3rd scorer. That's why I placed Horford, Butler and Millsap at the top of my list. They may cost more than TT, (who isn't even available, so I don't know why we're talking about him specifically), but they would give us more of what we need than he would. Whiteside is my 4th option because he would improve our defense and is an elite rebounder. Plus, he's more likely to be available than Tristan Thompson. Availability factors in at least as much as fit. Without it, my list would be LeBron James, Kevin Durant and Steph Curry.
BNM