Bingo. The reality of the situation is the Spurs would have been foolish to pass up on a talent like Fox who actually wanted to come to the Spurs, and they got him on a sweet deal while unloading some dead weight.
At that time, he was filling a glaring need and there is no way they could have reasonably predicted (or counted on) getting the number 2 pick and rights to Dylan Harper. And despite his Harper's good start, it's still far from guaranteed that he will reach the level Fox already has.
Yes, the fit is not ideal given Fox's extension...but it's not like the Spurs had a crystal ball. Anybody already talking about moving on from Fox after 17 games (and 5 total with Wemby) is putting the cart before the horse. Let's see what this team looks like together, give Harper a chance to learn from Fox without putting all the pressure on a 19-year-old rookie, and revisit the topic in a season or two if it's obvious there is not room for the trio of De'Aaron, Stephon, and Dylan.