JJJ isn't a "major star". Hell, you could argue he hasn't even been a real star this season. Averaging 19 and 6 on meh percentages, negative offensive EPM on a struggling team that didn't have enough scoring to begin with (red flag), 15 PER (career low), 57 TS%.
On the other hand, he's clearly a plus defender, which is obviously important, but plus defending isn't what pays you $50 million per year in this league. It seems the Ainges are gambling that the Jazz is a great fit for JJJ offensively and that he can rebuild his value.