They are alaready doing so. IPv6 test enviroments has been a disaster in the ISP I work for, too many devices don't support IPv6 in the domestic market and solution to router IPv4 over IPv6 are problematic.
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.
v6 with DS-Lite/464/MAP is going to be cheaper than v4-only because it allows the ISP to sell off (or not buy) most of their v4 addresses while also using less equipment. T-Mobile has already adopted this architecture.
Maybe you just don't get to know what problems it causes because they don't talk about it. Dual stack is currently under testing and afaik higher ups are not very happy about it.
Maybe we've done too early (it was about five years ago maybe, I don't really remember exactly) and now tooling is better, idk
Right now IoT devices use a communications model that overcome's NAT by tying the device to a service endpoint in the cloud. The device registers itself as an IoT device in aws and then your local hosts hit the device by going to the device endpoint in the cloud. I don't know if this model will hold up when IPv6 more widely supported though.