The Mavericks have failed to build around Luka Doncic for 6 years straight now. Kyrie isn't getting any younger, and he's playing as well as anybody in the league. He might not wait around for the Mavs to keep ruining his legacy if they fail to make the playoffs this year.
The Spurs know Wemby is great, and he is only going to get better from here, but he needs an elite passer to set him up. They have a mountain of assets from trading Murray to the Hawks, whose picks are valuable. This would be possibly the largest draft pick package ever, but if anybody is worth it, it's Luka. The Mavs would also be getting a likely top 5 pick in this years draft.
Mavs fans might not want to admit it, but maybe they need to start over since Luka is playing like an MVP and they still aren't really contenders, and they have very few assets to use to improve.