People here acting like Luka Doncic is late-career Isaiah Thomas out there on the defensive side of the ball. His D-DPM is -0.9, which while well behind his 90+ percentile skills is still 20th percentile among Small Forwards (which DARKO categorizes him as). He's been rated a positive defender by DBPM and DWS every year of his career, even if his offensive ratings are a lot higher.
Let's compare his career Darko DPM progression vs some of the peers being discussed here and elsewhere.

So he's ahead of those guys on overall career value (and would be behind James Harden, were we to include him). How do they stack up on defensive value, i.e. D-DPM?

Luka Doncic has improved defensively from earlier seasons and is currently rated above Lillard, and for the equivalent point in his career is ahead of Kyrie and Lillard and effectively tied with Donovan Mitchell (and before a late-season surge, was even with SGA too). Luka was also as high as -0.4 earlier this season before dropping off a bit lately - surely some team context may provide a partial explanation for him not playing to the same level he was mid-season.
Could he be better? Sure. As the above chart suggests, he has plenty of time in his career to get better, even Kyrie has succeeded at making himself a league-average defender of late, and Donovan Mitchell has rebounded as such. There are plenty of other players who are at least within shouting distance of Doncic's offensive value (4.6), let's say 2.0+, but are also positive defensive values: Jrue Holiday (3.3 / 1.6), Michael Porter (2.9 / 0.1), arguably FVV (2.0 / 1.5), Ant Edwards (2.2 / 0.5)... aaaaand that's the list. 4 guys.
Would we rather have 29yo FVV at $22M / yr than Doncic at $40M / yr (next year, and then 8% raises the next 4 years)? Yeah, probably.
Would we rather have almost-33-yo Jrue at $36M next year and probably $39M the year after? I would, but elite defense for 6'3 guards in their mid 30s is probably a tenuous thing.
So even if you're looking just at overall value rather than being bamboozled by POINTZ, there really aren't a lot of 2-guards offering a better chance of winning a lot of games than Luka Doncic. If I were Cuban, I'd much rather keep him, encourage him to work harder to his full defensive potential, and see what improvements he could make. He has a lot of career left, and fixing bad defense is one of the more achievable goals he could set for himself.
Let's compare his career Darko DPM progression vs some of the peers being discussed here and elsewhere.

So he's ahead of those guys on overall career value (and would be behind James Harden, were we to include him). How do they stack up on defensive value, i.e. D-DPM?

Luka Doncic has improved defensively from earlier seasons and is currently rated above Lillard, and for the equivalent point in his career is ahead of Kyrie and Lillard and effectively tied with Donovan Mitchell (and before a late-season surge, was even with SGA too). Luka was also as high as -0.4 earlier this season before dropping off a bit lately - surely some team context may provide a partial explanation for him not playing to the same level he was mid-season.
Could he be better? Sure. As the above chart suggests, he has plenty of time in his career to get better, even Kyrie has succeeded at making himself a league-average defender of late, and Donovan Mitchell has rebounded as such. There are plenty of other players who are at least within shouting distance of Doncic's offensive value (4.6), let's say 2.0+, but are also positive defensive values: Jrue Holiday (3.3 / 1.6), Michael Porter (2.9 / 0.1), arguably FVV (2.0 / 1.5), Ant Edwards (2.2 / 0.5)... aaaaand that's the list. 4 guys.
Would we rather have 29yo FVV at $22M / yr than Doncic at $40M / yr (next year, and then 8% raises the next 4 years)? Yeah, probably.
Would we rather have almost-33-yo Jrue at $36M next year and probably $39M the year after? I would, but elite defense for 6'3 guards in their mid 30s is probably a tenuous thing.
So even if you're looking just at overall value rather than being bamboozled by POINTZ, there really aren't a lot of 2-guards offering a better chance of winning a lot of games than Luka Doncic. If I were Cuban, I'd much rather keep him, encourage him to work harder to his full defensive potential, and see what improvements he could make. He has a lot of career left, and fixing bad defense is one of the more achievable goals he could set for himself.