Good stuff. Agree on all points.
It can't be overstated enough how important Gay is to this team. He's not a star player anymore but he's more vital to the Spurs than his raw talents would indicate. I think it's because:
1. Outside of White, he's arguably the team's best defender. His length and athleticism is disruptive. He's also really good at pulling down contested boards and allows the Spurs to go small without being physically manhandled in the paint.
2. Opponents not only need to defend him out to the three-point line, they have to keep a player with size on him or else Gay will feast in the low block the second he has a mismatch. Players like Forbes, Belinelli, Mills, etc. are all good shooters (better shooters, probably) but opponents don't have to worry about who is contesting their shots. It could be a point guard or a center and it has the same effect.
3. He's the pressure release valve of the offense. When things get bogged down and nothing else is working, letting Gay iso usually results in something positive. With Gay out, DeRozan turns into the pressure release valve and he's just not as suited for the job because there are matchups and defensive tactics that can bottle him up. Gay, on the other hand, has the length, high release point, ballhandling and quickness to get a good shot off in any matchup. Gay is limited by injuries and age these days but there's still not a player in the league that I'm aware of who really disrupts him when he goes one-on-one.