Demanding a trade and being a distraction is plenty of leverage.
He is under contract for 2 more years and he knows this - he also knows hes going to get paid and he knows that Dallas pays their own. All he can do at this point by holding out is hurt himself (financially, credibility, fan perception)....now were he on the last year of his contract, might be a different story.
He knows hes getting his extension later this year and rightfully so...ceedee will get his first, Micah will get his late in the year and Dak will get to go the FA market and someone can overpay for him.