Yes.
It's similar to asking "Should I still hold on to my partner who wants to leave?"
I'm of the opinion that the less time you waste with someone who doesn't want to be with you, the more time you have to be with someone who's a better match. No amount of bending over backwards is going to fix things.
What if Fox turns out to be the one that got away? Doesn't matter. He didn't want to be here as he doesn't think that he can succeed with the Kings. Let's respect his wishes and do what needs to be done for our benefit.
One thing to keep in mind is that Sacramento may not have the resources to commit to a super-max contract, should Fox actually make all-NBA. Remember, a lot of guys who might otherwise be picked will not be eligible due to the new 65-game rule. He is, at least by the numbers, having a superlative year, 13th in the league in scoring per game and 14th in steals per game.
San Antonio has only one player in the top fifty in scoring, one in assists and steals (Chris Paul), and of course, they have the leader in blocks. They desperately need help. No wonder Popovich (reportedly) attended practice today. It is a team that should be amenable to a cutting a good deal with Sacramento. As the saying goes, you have to strike while the iron is hot.
Apart from that, Malik Monk has shown he can be the lead point guard for this team, despite his inconsistencies, especially when paired with Keon Ellis. The lineup of Sabonis, Murray, DeRozan, Ellis, and Monk currently is +2.9 with a Net Rating of 23.2; compared to +0.8 and a Net Rating of 1.8 for Sabonis, Murray, DeRozan, Monk, and Fox; or +1.7 and a Net Rating of 20.4 for Sabonis, Murray, DeRozan, Ellis, and Fox.