Well, it sounds like they simply didn't want to think long term. They are basically 70 and 80, so it's kind of understandable. Maybe they wanted to just sell it, divide the money among kids and grand kids, save them the headache. Who knows.
I think those rights might be worth more than any NBA team. You have an endless stream of increasing guaranteed profit. It's basically 2% of NBA's TV and online broadcast revenue, in perpetuity, and the NBA is about to sign new more lucrative TV deals in a couple years. 500 mil is a lot, but those rights would likely bring that 500 mil in 10 or 15 years. And more than that in the decade after that, and more after that, etc.
If they were still 50 years old, they probably wouldn't settle.