Also, MOST stadium use a sole food providers and the vendors are just partners. There are a couple of stadiums I know that use dual food provider it simple doesn't work well and can only work in a modern facility. Having multiple food providers require different kitchens, storage areas, shipping centers, staff, as offices. Bottomline, it's a logistical nightmare, and with the stadium being as old as it is, there is no way this would be possible.
Also, most of the upgrades done during the Disney area were improvements that only the fans can see and most of the electrical, plumping, and networking issues still exist. For instance, the reason why the stadium has beer bomber stations isn't because it's what is better, it's because the infrastructure will not support more beverage, power, and CO2 lines.