I never thought Dallas would tank. They won't be having a firesale. They are going to be as good as possible while Dirk and Carslile are there. That should be obvious.
This is what I think also, Carlisle and Nowitzki have always been out to win every game they can so they can make it to the playoffs where, as long as you have a player like Dirk, anything can happen. Yes, I know Dirk is old and unable to be a superduperstar anymore, but I am sure he and Carlisle don't see it that way.
I also don't think Cuban would have anything to do with purposely making his team worse to get a better draft slot.
For those reasons I don't see a major trade off of players or overt tanking. Dallas is more likely to make a trade like they did for Rondo than they are dealing anyone valuable for lesser talent.