It's all about talent. The NFL is only where it is because when an athlete laves college and goes into the draft he naturally goes into the NFL draft.
The USFL or any league for that matter can survive if the owners are willing to make their league a huge money pit and back up ridiclous rookie contracts at the draft then afterwards poach free agents ever year with insane contracts. When the NFL is stocked with aging athletes and D-II scrubs and terrible play while the College heroes and really good players are doing there thing in the USFL, congragulations you have a new default league!
At that point maybe the dying NFL merges with the USFL losing some scrubby teams at the time like Arizona or Seattle. The Big boys like the Packers, 49'ers, Raiders, Steelers, Chiefs, Cowboys, and Dolphins would survive but the middle of the road and bad teams might be replaced by USFL teams.
Of course this is completley disregarding stadium deals, TV deals, fan loyalties, and deals with a cities sports commision and tax payer funding and all the money issues that keep the NFL so firmly entrenched as the big boy football league (same goes for MLB with Baseball and NBA with basketball)