Luka is better offensively than AD but AD is ofcourse better defensively. What Nico might be thinking is AD right now is a 25ppg player playing with a ball dominant LeBron, maybe putting him at the center of the offence can potentially make him a 30 ppg player while keeping him great on defense. Regarding the future, it's clear that the future was going to be difficult with Luka as well given they were severly limited in their squad building activities. They already gave up their future to bring in Kyrie and were in a 'win-now' mode already, they just believe this trade gives them a better chance.
Again, there is also the possibility that Mavs didn't want to give Luka the Supermax and knew they eventually had to trade him. I suppose the logical thing to do then is open trade offers, where they could get a ton of draft picks back, but it seems they only wanted AD since they didn't want to lose the 'win-now' situation given they have a decent group and Kyrie.