The copper between the houses won't last forever. Eventually higher speed transmission media will be available. Narrowband ISDN was basically a stopgap measure. But it's development allowed time for the design of Broadband ISDN to be even more ambitious.
Broadband ISDN (B-ISDN) is similar to Narrowband ISDN only in the most basic aspects. The reference configurations are largely the same, but every bit of the underlying design will be replaced. The S/T bus will by necessity have to be a much faster network, as will the network at the U reference point. The signaling will be changed, as will most other details. The house will most likely need to be rewired yet again, and all the physical connectors are likely to change.
B-ISDN is very strongly related to ATM. This is because ATM will provide a consistent data encapsulation scheme that can be used throughout the network, starting with your TE1 or TA equipment, and covering every piece of telecommunications equipment in use.. ATM is so important to B-ISDN that many people believe it's the same thing. It's most of what is new over Narrowband ISDN, but it isn't all of it.