That is an extremely misleading stat.
The switch to Amazon is what caused the massive drop-off in numbers. When Amazon took over, TNF decreased in viewership by over 30% immediately. If we look at the pre-Amazon years, 2018 to when Amazon purchased, TNF numbers averaged a little under 14 million viewers (13.3 to 13.8).
Last years TNF numbers were only a small increase (approximately 10%) of 2019's TNF numbers. In other words, when all the other recent stadiums were being opened, TNF had very comparable numbers to what they have now (the big increase is only in relation to the huge drop they originally caused).
Sunday Night Football is what the vast majority of other stadiums opened on. SNF averages 53% more viewers than TNF (Even with amazon).
I have the full spreadsheet of viewership numbers if anyone wants it.