Your point is quite valid. If you treat Netflix like cable TV, and you watch TV the way many consumers do you'll likely get close to the current cap Comcast has. However most people that are Netflix customers that use their streaming service don't use Netflix in this way.
Today most people that are going to watch that much TV already have cable or satellite tv service, or are happy with over the air tv. If this changes either Comcast will need to increase the cap, or make a deal with Netflix and other video providers to get part of the subscription fee and not count the data against people's data caps.
The most important thing to recognize here is that most consumers don't want to think about bandwidth, setting up a box, anything particularly geeky (even watching something in a browser is too much). We're likely to have the most hours viewed (and dollars paid) in front of TVs that take care of everything automagically; all you have to do is keep paying your bill every month.
The long tail of content will likely be cheap (around what you pay for Netflix streaming today), but the cost of Netflix is likely to balloon once people start watching mass market content en masse, leaving their "TV"s on all day, etc. These people will pay for service. They already do. They aren't cutting the cord because they can get somewhat obscure documentaries on Netflix while the local cable provider sticks with reality TV and sports.
The end game that I see here is that most consumers are going to pay around $30-$50 a month for "TV" that covers the costs of bandwidth and a lot of popular content, while movies in theaters and sporting events are "on-demand" or something similar. The only question is who controls it.