I understood what you were saying. I just don't agree with it and don't believe there is any evidence for it either.
Attendances have been declared based on ticket sales rather than the number of people who actually turn up for many, many years. If there is a difference now, I think it's that season tickets have become more attractively priced at many clubs (including Cardiff City). Therefore, there are more season ticket holders than there used to be and such people won't always turn up for games.
I've noticed that's very much in the case in my local, where there are definitely more season ticket holders than ever before and the declared attendances reflect that. However, as we know, plenty don't turn up for every game.
If you look at City's attendances for equivalent Saturday home games at this time of year during the last five Championship campaigns, the figures are as follows:
2014/15 v Wolves = 21,165
2015/16 v Preston = 15,566
2016/17 v Fulham = 15,656
2017/18 v Middlesbrough = 18,720
2019/20 v Brentford = 22,393
Just like 2014/15, when there was an increased number of season ticket holders following relegation from the Premier League, the figure yesterday was larger than average for this time of year. I don't know what the actual physical attendance was yesterday, but it looked to me to be around the 16,000 to 17,000 mark, which would be bigger than 2015/16 and 2016/17, and slightly down on 2017/18 when the side was challenging for automatic promotion.
From my perspective, those figures all look to be about right and don't indicate that this season is one of a decline in attendances. In the Premier League there were more than 52,000 at Newcastle v Burnley yesterday, while the attendance for West Ham v Southampton was just short of 60,000. That doesn't suggest to me that the 'football bubble' is bursting.