The USFL was already in trouble by 1984 financially with most if not all of their clubs losing money. And this really didn’t have anything to do with fan support. The fans were there for this league to still be in business as a spring/summer league today. But the league simply grew too fast with eighteen clubs by 1985. And most of these clubs being in established NFL markets. Like Washington, Pittsburgh, Philadelphia, Detroit, Chicago, Tampa, Dallas, Houston, Denver and Los Angeles.
I’m not sure the old USFL could’ve been saved at this point, but moving their games to the fall which was the plan for 1986 was probably the thing that killed the league. Going up against the NFL in their markets in their territory the same time the NFL is playing. Perhaps the only thing that would’ve saved the old USFL at this point was going back to the old spring/fall schedule. And contraction or relocation of USFL clubs that were in NFL markets.