The trouble is, there was no Fox-for-Sabonis trade to be had. The Pacers were interested in Haliburton. Fox's value wasn't particularly high at the time of the trade, but even if it was, your trade partner has to want the player you want to part with. There was no indication that the Pacers were interested in Fox. The Kings could have eventually moved him for a package that got them less than the All-NBA big they got in return for Haliburton, so I guess you have to decide if Haliburton/?? would have been a better possible future than the Kings present of Sabonis/LaVine/Monk (who was attracted to Sacramento in part because of his connection to Fox).
In the end, you're probably not a contender with either Fox or Haliburton as the centerpiece of your team. Hali's younger, though a worse defender, and they both inexplicably disappear on their teams for stretches. Star talent needs to be dependable in the NBA. You have to be able to count on it. And I don't really believe in either player as a dependable star that can carry their team over the finish line with consistency. Fox has that "it factor", and honestly, so does Haliburton. So it's weird that neither of them can pull it out of themselves with the necessary regularity.