Let us not forget that Dallas are winning even without KP, maybe they can trade him in the future to get a more durable (not prone to injury) player to play with them and help them to improve their defense, Luka needs some big help as he can't handle the big teams alone come this playoffs.
He's supposed to be a good trade asset on the team but for his current situation many team ownerd will dou t about his health since they might not use him if he will be injured again although his a good caliber player still the mindset of team evolves now mostly teams looks for young healthy stars to be added on their rooster.
Other teams know how much of a injury problem Porzingis is as well, if he wasn't this much injured he would have been a great asset but we all know he is. If Mavs ever want to trade him, they are not going to get too much for him, sure they are going to get some, but not too much. This is why I think they are keeping him for now, maybe they will drop is contract because of all the injuries and in that case they could get a lot more chances to get something. Even few first round picks from a low level team that is tanking would be insane, they could get someone from draft that will compliment Luka and they could play for a lot of years.
In any case, having Luka is a problem, why? Because he is good enough that he carries the team to playoffs so missing good draft position, but he is not good enough to get a championship all by himself, this puts Mavs in a difficult position.