People still listen to opera too, all of those three old guys in New York and San Francisco. So what? How many of the under 30 crowd listen to jazz? 1%? Jazz is part of an old geezer generation even in the USA and it is even worse in other countries. It is something the overwhelming majority of young(ish) people have no emotional connection with. This name is really, really bad long-term. And, as I said, it is really hard to market for merch.
And that's OK, the world is changing and we don't have to have our teams named the Utah Halberdiers or the Chicago Gentry. The names should be cool and attractive to new generations of fans, don't you agree?
And, anyway, the name "Jazz" is not associated with any past glory. Rather, it is firmly tied to probably the most universally disliked NBA player, Karl Malone. A clear break would be awkward at first but clearly beneficial in the long run.