What you're recommending won't be very effective or useful:
1. ISPs do not offer real Quality of Service guarantees because punters don't want to pay for it. Your recommendation would dramatically raise rates.
2. Requiring network capacity to support the 60-80% usage mark for all users would require a huge amount of unused capacity sitting around, so there's a really idiotic amount of waste incurred that no ISP can afford to provide.
3. ISPs cannot and will not offer QoS guarantees for other ISPs, i.e. they can't ensure that some other ISP won't introduce packet loss that makes your connection suffer.
4. Service levels really don't just depend on local loop utilization. Given all the various L4 connections make between systems, it's certainly very difficult to ensure some sort of uniform performance guarantee with only one component of many in the end-to-end connectivity that exists with the Internet.
Network capacity planning is complex, but exponentially so when you have so many individual ISPs all interconnected to provide "Internet" services. Trying to regulate a minimum performance level with such random, aggregated connections is virtually impossible, since no single person understands all of the connectivity end-to-end.