Football attendances took off in the 90’s with the introduction of the Premier League.
On average the top flight has seen attendances increase by ~100% over the last 30 years, second tier teams by ~70%, third tier teams by ~50% and fourth tier teams by ~25%. The biggest increase happened roughly between 1992 and 2002. A period where we happened to be languishing in the lower leagues and Derby and Coventry were Premier League teams for almost all that time and Ipswich spent half in the PL and half in the Championship.
In 1992, the average attendance in the Premier League was ~20k, in 1997 it was ~28k, in 2002 it was ~35k. Last season it was 38,500.
If you were a top flight team in 1991, you won the proverbial golden ticket when it comes to future attendances.