While I agree that the base for an NFL team is likely not there in SA because it is such an economically depressed area, the Spurs don't seem to have any trouble filling seats. They even managed to get out of the Alamodome and into the SBC Center.
There are some medium and large companies based in San Antonio: SBC, Clear Channel, KCI, Valero, USAA, Tesoro, Titan Insurance...but not many. Plus, the TV market doesn't look that appealing with Dallas and Houston siphoning off so much the fan base in eastern Texas.
Cornu taurus petit!