1) Damian Lillard
Last but not least is Lillard. While the Spurs were initially on Lillard's wishlist, it became clear that he preferred the Miami Heat. Despite that, he ended up with the Milwaukee Bucks and was acquired for a haul that the Spurs could have beaten.
The decision not to aggressively pursue Lillard could certainly be second-guessed since he would have given San Antonio an elite offensive weapon that would have boosted their 28th-ranked offense and kept teams from honing in on Wembanyama.
Lillard is much older than the Spurs' young core, but after a slow start to this season, he has started to look like his old self, quieting concerns that he is beginning to decline. With three years left on his contract, he would have given San Antonio more than a stopgap at point guard, added more shooting to the starting lineup, and likely made life much easier on Wembanyama.
Unfortunately, the team chose a far more unconventional method for solving their point guard problems by shifting Sochan to the one and bringing Tre Jones off the bench. That's one way to address their biggest need, but Lillard would have been a better option.