Or we end up giving San Antonio the #2 pick in the draft and end up drafting 25.
extremely, extremely unlikely, for a host of reasons
the fact is that if you project out 4-5 years or more, the overwhelming likelihood is that the picks for any two given teams are gonna both fall in the 10-20 range, particularly if the teams in question have a history of being reasonably competent.
it's easy to think of worst case scenarios, but there's a very good chance (like probably >50%) that the Celts didn't actually give anything up with that pick swap (because the Spurs will have a higher draft slot that year anyway), and a significant chance that even if the Spurs' pick that year is worse, the Celts will only drop back a handful of spots.