Extending guarantees another wasted year in Utah without any say of his future unless he becomes a diva à la Harden, which he won't. He's not young any more, and his prime years can be over very soon.
I couldn't care less how many hundreds of millions he will make during his career, he and his family are set for life and he knows it, it's purely down to basketball now. Writing an extension gives Ainge full authority to do whatever he wants to do with him, if he wants to sell his soul for money and be remembered as another one of Ainge's chess pieces, go ahead.