We can make up any numbers we want to put the blame on one side or the other. It could be the case that either side was being completely unreasonable, it's all BS until we actually know the numbers. But it's pretty simple logic to say that if Kessler plays well, he's going to get more than what we could have extended him for. If he plays poorly, he's going to get less than what we could have extended him for. Kessler bet on himself, the Jazz bet against him.
And no, I do not believe in this "cap space preservation" thing. I am in full agreement with
@Handlogten's Heros that this a dumb explanation. Long term financial relief from a front loaded contract is significantly more important.