Comcast promises a certain amount of download capability. When customer requests for content that originate from Comcast's network (that would not normally exceed that download capability) end up saturating a link to which Comcast is party to, then it is Comcast's obligation to attempt to upgrade that link. Comcast cannot control the routes via which the content is returned, they must react to their customers usage patterns to improve the performance of their network. So if tomorrow, everyone wants to download content from some other peer then Comcast will need to upgrade other peer connections.

The exception to this is if Comcast wants to charge rent to the internet to have access to Comcast customers. Then it is in Comcast's corporate interest to let peer connections saturate and then charge upstream content providers rent to remove the congestion.

A pretty ballsy move for a company (who's assets are stretched out over thousands of miles and in plain sight) to attempt to extort the entire planet. ;)