Depends on timing and wishes, no?
I don't think its hard to imagine a situation in a yr or two where Fox, Castle, and Harper all excel and all playing less minutes than they should. Fox sees a situation he likes where he doesn't have two much younger creators/cutters taking his minutes and reducing his role.
When Fox requested the trade here 2.5 months into the season Castle was much more of a question mark. Getting Harper was pretty unforeseeable.
Smart teams trade high-paid, high-impact players when they have multiple, younger guys with duplicate skill-sets and an imbalanced roster. Smart players and agents request trades to better individual situations (like Fox did already) if their concerned with maximizing their individual impact and/or stats and/or future success.
If Castle and Harper develop abnormally fast, and Fox is playing less and/or has a lesser role than he wants as a 28, 29 y/o its completely rational for him to be happy to move to maximize his talents and the Spurs will go out of their way to have him land where he wants. In that easily foreseeable scenario, it would make the Spurs a more attractive FA destination not a less attractive one.
The #2 pick changed the landscape, it wasn't a reasonably foreseeable event for Fox, Klutch, or the Spurs in January.
If Harper lives up to his potential, him and Fox will pretty much always be a sub-optimal fit, both should be primarily on-ball guys, both will have to sacrifice with the other on the roster. One plus Castle is great, but hard to imagine all 3 ever maximizing synergy together.