I agree with much of what you're saying in your post Steve, but the period in which City were getting 25,000+ crowds on a regular basis in the second tier was actually relatively brief.
The season averages for the years concerned are as follows:
66/67 - 10,259
67/68 - 13,301
68/69 - 16,870
69/70 - 21,502
70/71 - 21,522
71/72 - 15,510
72/73 - 11,456
The first Cardiff City replica kit I owned was in 1975/76. I remember being the only kid in my year at school who had one. The other kids who had replica football kits were wearing those of Liverpool, Manchester United, Leeds and Chelsea. Obviously that was Sky's fault.