Comcast themselves have even admitted
Why are you trialing usage-based billing?
The Internet ecosystem is changing constantly and we decided back in May 2012 to replace our static 250GB usage threshold with more flexible data usage management approaches that offer more choice, flexibility and fairness for all customers. Customers can choose to use as much Internet as they want, and those who choose to use more pay more, while those who use less can pay less. The vast majority of ISPs, large and small, have some version of data usage plans in place.
You are right, when everyone is using the pipe at the same time, there will be degradation. So why not charge for internet access like electricity? Make it cheaper during non-peak hours and convert to fully usage-based billing. No flat fee for access, or at most a very small one.
To answer my own question here with part of your comment: it would be really confusing to customers.