Negative - Spurs lose even more three-point shooting
This one is the elephant in the room. Already ranking dead last in the NBA in three-point field goal percentage, San Antonio dealt away their most pure three-point shooter. Their league-worst team percentage of 34.1% in the current season is almost certain to decrease even more after losing McDermott.
One big positive to take from this is the fact that the Spurs are not in the neighborhood of pursuing a play-in spot this year. If they were perhaps on the cusp of the ten seed and could potentially get there with an increase in shooting, this would have been a questionable decision. But with them clearly thinking long-term, I think this is a fine move.
The team will want to continue having shooters around Victor Wembanyama, but McDermott was about as much of a three-point "specialist" as you can get. His game was extremely one-dimensional, and the Spurs can now focus on signing players that can shoot at a similar clip to McDermott while having more developed games overall.
McDermott was also realistically taking shots away from other players that will now get a chance to improve their percentages just by taking more threes in-game. Becoming a worse three-point shooting team now could mean becoming an overall better one later.