You're assuming there is only one internet connection in my home network, for example. The "easy" trick where your ISP gives you routable addresses does not work when there's more than one exit.
Still want to help? :)
And really... everyone is pushing for SSL everywhere - among other things so that the ISP doesn't MITM your traffic.
Why would you allow the ISP to know what machines are inside your home network then?
This doesn’t change anything about the NAT or firewall story, and having two different connections is complex with IPv4 just as well. Aside from being a fairly exotic setup for personal use anyway.
What would your ISP do with the information that there are 73 unique addresses in your network at this point in time? Especially given that devices may mint any number of them for different reasons, so you can’t even really assume that corresponds to the number of physical devices in your network?