Easily debatable. It's not TT's fault he got drafted to an abysmal Cleveland team while DD fell onto a perfect team for his skillset. Dale Davis played multiple seasons with a league leader in assists a game (Mark Jackson), HOF wing in Reggie Miller, another multiple time all-star in Detlef Schrempf (sidenote: loved his cameo in Parks), and played most of his career next to an all-star level offensive center in Rik Smits. Tristan's played with Kyrie (great but still improving) and a 1/2 season with Luol Deng. You don't think that explains the discrepancy between their efficiency?
And before we forget- DD broke out during his age-23 season, Tristan has yet to finish his age-22 season. Let's see Tristan play his age-23 season before calling DD the superior player.
Terrible assumption. He easily has room to grow multiple aspects of his game, and he's shown the work ethic needed to consistently improve incrementally. Him not improving at all is about as likely as him improving to a 20 game a night scorer, which is to say they're both highly unlikely to happen.
Haslam's not a bad comp. I think TT's already at the level of a valuable big getting 25 mpg on a contender (which you somehow keep pushing as his best case scenario)- look at the caliber of players in that very role- it's not exactly difficult for a player to reach this.
Brooklyn - Andray Blatche (22.3)
Charlotte - Anthony Tolliver (21.3)
Dallas - Brandon Wright (18.0), DeJuan Blair (17.0), Samuel Dalembert (19.9)
GS - Jermaine O'Neal (19.8)
Houston - Terrence Jones (27.5), Omri Casspi (18.1)
Indiana - Luis Scola (17.2)
Memphis - Kosta Koufous (17.8)
Miami - Chris Anderson (19.4)
OKC - Nick Collison (16.7), Kendrick Perkins (19.7)
Phoenix - Markieff Morris (25.9), Marcus Morris (22.4)
SA - Boris Diaw (25.2), Tiago Splitter (21.1)
Washington - Trevor Booker (21.1)
IMO none of those players are as valuable as Tristan right now. You really believe it's Tristan Thompson's best case scenario to end up playing this role? You really see no possible outcome with him developing into a starter on a playoff team?? I think by assuming Tristan's done improving you've capped his growth, thereby making your projection of his ceiling completely moot.