I think you got that the wrong way around. Old analog and ISDN connections are specifically mentioned to never get v6 (which makes sense, I guess). Only new contracts with a fixed IP will definitely get it.
But... Why does it make sense?
Alternatively, ISDN/dialup gateways just don't support IPv6 because they were all designed in 1995.
Here you are supporting the network problem. If no ISPs support IPv6, it makes no sense for websites to support it. If no websites support it, it makes no sense for ISPs to support it.
You're right that everyone with an address needs no new address to be globally reachable, but in order to actually fix the issue we need everyone to switch. The protocol versions just don't inter-operate.