All people have to do is speak with their wallet. If there was sudden mass migration from Comcast over this policy, they would surely pull back on it.
Problem is, people will piss and moan, but how many will actually leave Comcast over it? Anyone? Probably those who have U-verse or FIOS in their service area, which would likely amount to upgrade anyway.
How many will take a stand and go back to DSL,Wireless, Satellite or Dialup in order to send that message to Comcast?
cue the chirping crickets.
Yet, if Comcast does get away with, without significant subscriber loss, other providers will likely follow suit.
About a year ago I got a message from Insight (before they switched to Comcast in my area) saying I'd been using too much bandwidth for the past few days saying if it continued they could limit my access. They cited some part of the contract where it mentions knowingly hurting service for other subscribers as justification for their actions.
So, I decided they blew their chance with this long-term customer. I had already dumped their TV in favor of Dish a decade ago, so I called up AT&T and asked what they could do by bundling our home phone, cell phone and internet. They saved me about $30/month which was close to what I was paying Insight. I dropped from 10MB down to 6MB down, but, honestly, I don't miss the extra speed.
If enough people would drop Comcast in favor of some other service, it might make them see the light of day.