When you are talking about tens of millions of customers, it's not feasible
to give each a subnet even if you have the space to do it.
Why on earth not? You give them one "thing" either way. Same
overhead to issue, same overhead to route.
There is nothing to indicate that ISPs are going to change their business
models simply because IPv6 address space is plentiful; they charge extra for
two hosts because it is assumed two hosts consume more bits than one, not
because a second IPv4 address is hard to come by.
This is not the least bit believable.