Today, dial-up concentrators usually have an address
range that it used to assign addresses to people that dial in. That
means at most a handful of routes per dial-up concentrator in the
interior routing protocol. If everyone has their own /48, that means a
route in the IGP for each customer that's online. There are no hard and
fast rules about how many routes you can have in an IGP, but somewhere
between 10k and 1M you run into trouble.
this is an interesting point, but I think it has more to do with whether
the prefixes are statically bound to customers than the length of those
prefixes. why would giving customers static /64s result in fewer routes in
your IGP than giving them static /48s? in neither case is there a direct
correspondence between the customer's address and the concentrator.