cap the MAX SPEED? > That's why I'd cap the MAX SPEED for those who would be willing to go with less at peak times.
The technical limits of the cable system (i.e., coax cable that attenuates frequencies above 1 GHz) provide your suggested cap of MAX SPEED. The limitation is included in the design of the system. In old 300 MHz systems, an option for doubling bandwidth was to run two coax cables to everyone. Today, two options might be to: A) eliminate the hundreds of TV signals coming into our house (except the one or two that someone is actually watching) to make room for more data bandwidth, or perhaps B) replace coax with fiber.
I wouldn't expect system capabilities or design to be affected by some percentage of people agreeing to a speed cap. That's just going backwards into bogus schemes that make the corporations a few extra million bucks.
Build the infrastructure, free the bandwidth, and occupy comcast.