I think it's pretty clear that Porzingis is the problem wherever he goes.
Doncic has made THJr much better. That Mavs team is blah on paper, Doncic elevates the whole team. But I do think Luka has the similar LeBron "issue" of being so good that supporting stars simply have to take a reduced role unless you're a great iso-scorer. Bosh, Love, etc. That's exactly what KP is in DAL: a worse version of MIA Bosh or CLE Love. A floor-spacing catch-and-shoot big. Go stand in the corner. KP's offensive repertoire is more limited than Bosh or Love though. He's 7'3 in his 6th year in the league and still terrible in the post. It is strange to me that a Luka/KP PnR game isn't nearly unstoppable.