Why didn't they ever upgrade ISDN to ADSL speeds and beyond, or designed an ISDN-over-PON? Was it cost reasons?
If someone from Germany can chime in, I believe they had higher-bandwidth ISDN services in the late 1990s onwards.
You could go to E1/PRI to get 2048kbps for €€€€.
In North America ISDN was 56k due to the slower T1 it was sent in and framing.