Colorado was in the WS in the somewhat recent past. It might be a bit unfair to declare their organization as incapable of success forever into the future.
While I agree that generally it can be annoying seeing so many fans from other teams in AZ sports arenas, it comes with the territory. AZ has proven that if you put a competitive team out there, people will come and road teams won't have home games here. The Cardinals are a perfect example.
The D-backs should be thanking the heavens so many people live here that root for other teams. If not, they might be playing in front of like 6 people.