I can't think of a city that identifies and loves it's team more than SA. Spurs fans are pretty die hard. When the team was rolling Spurs stuff was everywhere during playoff runs. Bars full, people going nuts. I'm not going to denigrate the local SA Spurs fan.
The lower bowl is completely segregated from the rest of the arena. It has it's own concessions (and seat service), it's own restrooms, etc. Completely self contained, and you can't get down there without passing a couple of ushers who check tickets. Those seats mostly aren't the big fans. It's the rich fans and corporations that have season tickets but only use them for hosting business guests or employee perks. Usually they'll sell the ones they dont use, but I could see them having trouble selling right now simply because the economy is really bad in the US right now. SA is a blue collar, working man city. It isn't an über expensive tech hub like Austin, a wealthy melting pot like Dallas, or even the big oil money city like Houston. It usually is stabilized by the military and tourism during economic downturn, but so far that has seen a slow down from previous years numbers.
I haven't yet been to a game this year so I don't know what the upper bowl looks like. Anyone who has been to these two can share if it's mostly full or also lacking a lot. But I do know a lot of SA families are struggling to put food on the table right now. When that's the case it becomes much more attractive to watch on TV for nothing than spend $$$ to get to the arena.
I'd also say keep an eye out for the rest of the season. This team has been bad for years now, and this is just the beginning of the rise up. If the team shows it's real as the season goes on more and more people will bite the bullet and get to the arena.