Honestly this is part of the problem with the Spurs being in no-mans land. It makes decisions like these so much harder/far less obvious. If we were at .500 and in the 9th seed, then it's easier to pull that trigger. If we were a bottom dweller with top lottery odds, it would be easy to say no. Instead we're in that middle space with a superstar too good to let us be worse, but a roster too bad to let us be better. Trading for Fox makes us better, but so does having a Top 4 pick, and if we ended up with somewhere around a 40% chance to get one via the Hawks and Spurs pick... that's tough to forgo.
I'd personally try to make the move for Fox that somehow allows us to keep the better of ATL/SA at a minimum, but I'm not sure that pathway exists before the deadline. But I may also be overestimated the Kangz FO.