What's important to understand is the player power. With his contract extension, Luka could dictate where he could go and how much the Mavs got.
The real issue here is why did they decide to trade him. Once they did though, I do understand both the deal they got, and what the situation is.
Now what I don't understand is why they came to the conclusion that trading a top 10 player in the NBA at 26 years is the right move and giving him 350 million is the wrong move. I have no reason to believe Luka is not worth that money based on how he plays on the court. Yes his defense sucks, but so does the defense of many superstars who get maximum contracts. Something went on between Luka's team and Dallas and something they saw about Luka must have really convinced them he's never winning a ring as the leader of the team. So they made that decision.
Back to when the decision was made, once that was made. I think they would have had only those 2 options. Trade a disgruntled Luka to a team he would force them to trade to and get assets either worse than the Lakers deal or get a complete re-build package. The issue with getting a rebuild package like most wanted from a Luka deal is that you have Kyrie, Gafford, Wahington and many more making "win now" money and type of team that is in "win now". You probably have to then scrap the entire roster and only keep Lively and go all out re-build. I am not sure they had the draft picks incoming or the means to do that (or did the owners maybe want that?).
As I said I've seen players like Shaq and KD get traded. I've seen many great players get traded while they could still give a team a championship or 5 more years. I have never seen them get back a player who was an all NBA player. This is the first time this happened. So I don't think the Mavs did bad in that regard, if their objective is to try and win.
Problem? Kyrie tore his ACL. He may not be himself again, and he may not even come back next season. So now unless you replace Kyrie with an all-star guard, while it's nice to have AD/Gaff/Lively and all that size/defensive potential, in the end you're a play in team unless you get a Kyrie replacement.
Honestly our Suns are in the same boat in many ways. We just don't have that kind of franchise star in Booker. We should absolutely be looking to sell Booker off for a similar level player and some future assets. If the Lakers had given us AD, a future unprotected pick and a young starter I'd jump on it. They never would. Maybe some team will give us something comparable, and we should definitely take it. We can't rebuild for a while, so we need to try and win with the pieces we have until we get a situation where we are 1 move away.
Even if we trade Durant, we won't get nearly as much as we gave. We have to try and stay competitive because the team's draft situation is such, we'd give away great lottery picks. Honestly, I don't see how Booker helps this team. He's neither a point guard that you can build your offense around. Nor is he an elite defender you can build your defense around. He's a really good secondary weapon to have when all your other pieces are in place. Well, KD is the exact same thing. Anything Book does for a good team, KD does as well. Might as well try to shop Book and get a defensive piece or a point guard that can run your offense or just plain get more starting level depth. You certainly won't get that with trading KD or Beal.