The best case scenario for the Mavs is that their core group of vets shows a high ceiling this year, but they ultimately pack it in, veteran talent on the roster notwithstanding, and manage to squeak in under than top 7 threshold.
Then, they use that top 7 pick plus their cap space next summer to retain Deron, Wes, Parsons, and Dirk while adding significantly to that group, and the following year they return to the playoffs and end up giving up a pick in the late teens -- the sort of pick they rarely use and develop as a franchise, anyway.
Maybe it's outrageously pessimistic to think that could come to pass, but as I've said, we need only look a few years back to see what happened with that Warriors team. It's not like that Warriors team was lacking in talent, though it was concentrated entirely on the offensive end, much like the Mavs.