To the best of my knowledge, Comcast is running dual-stack v4 and v6. The GP was talking about running a purely v6 network, and pointing out that it wasn't yet feasible. Your example of Comcast doesn't really fit the bill because Comcast already has a v4 network to all of their customers, and they are not migrating customers to a solely-v6 network.

This is the chicken-and-egg problem that all new networks are facing with regard to IPv6 adoption. In order to have a usable network, you have to support IPv4 to all endpoints. But once you have v4 at all endpoints, the incentive to run v6 is greatly diminished.

As always, v6 needs a "killer app" that Grandma wants to use that is unavailable over the v4 internet, and then network administrators could use the actual demand from their customers as a justification for moving to v6. Unfortunately, at the moment, the list of v4-only must-have apps is still greater than the list of v6-only must-have apps.