The Packers' last season at the original City Stadium was 1956. That place seated only 25,000. New City Stadium, later renamed Lambeau Field, opened in 1957 and originally seated just over 32,000. Some of the AFL stadiums had smaller capacities (e.g., Frank Youell Field in Oakland, and Nickerson Field and Alumni Stadium in Boston), but I'm pretty sure no NFL stadium did.
The Chargers would be in deeper trouble than anyone thought if they couldn't sell out a 30,000-seat soccer stadium.