I think this would work better for personal trainers, to basically be paid on your improving performance. But gym pricing relies on those who don’t show up to use it to subsidize those who do. So if you are the type of person who will use the gym, you benefit from others paying but not going. And of course everyone thinks they will go when they sign up for a year.
If it were priced in the way the post explains, it would be far more expensive for those who actually use the service.