You have it backwards. There needs to be opportunity there BEFORE a stadium is built. If there was opportunity there, other businesses would already be investing in the area. There would already be people in the area. It's not an if-you-build-it-they-will-come scenario. A new football stadium wouldn't suddenly attract people back to the region.
Instead of a big expensive building barely being used out in the suburbs, you'd have an even bigger, more expensive building barely being used downtown. Oh, and on top of that, the football/tailgating experience would suck now too.
Some people think "drop a stadium downtown and now Buffalo will be like Nashville!" and that is just wishful thinking.
Nashville was Nashville first. LA was LA first. Atlanta was Atlanta first. The Twin Cities was the Twin Cities first. These places were not built by NFL stadiums. As such, Buffalo will not be built by an NFL stadium.