Now that football popularity is off the scale the factors years ago that determine crowds are less relevant. What you are saying was certainly correct years ago. From memory, Boro averaged 30000 in their first season in the top flight under Big Jack and then as the novelty wore off crowds dropped by 10000. Coventry and Birmingham were averaging 34 and 36k respectively when they went up in the 60s and 70s to place them in the top six, before dropping to around 12 or 14k. Coventry were drawing huge crowds as a Third Division club in the sixties.Attendances are funny things. It's not always the higher the league the bigger the crowd. Mediocrity can set in, even in the PL, and crowds dwindle after a few seasons. Nothing like a promotion campaign or just a general feeling around the town/city for a team etc. to get attendances up.
In 97/98, in the second tier, we had a season ticket sell out. 30k average (before expansion) and probably for some games would have got over 40k. But then when we got comfortable in the PL, under McLaren, the same enthusiasm wasn't there.