The Patriots would only draw well when the team was good. It wasn't until the Parcells-Bledsoe era did they start selling out every game. Even if they do go back to being a bad team I think they've established themselves now so they'll still sell out their home games.
That was my impression as well. When a team builds a winning tradition, then the fans will keep coming. Teams that have been mediocre forever aren't going to sell out no matter where the team is located.
Denver has been very competitive for the last 20 years.
And the area is more into baseball and hockey, so football might flag a little if the team reverts, but I think they're starting to really establish themselves in the hearts and minds. They'll be alright.