I don’t remember the PSTN ever being attacked
Prior to deregulation of the phone exchanges and last mile, the PSTN was accredited to carry information at “Secret”.
One of the main objections for allowing non bt people and kit in exchanges was that it’d stop that accreditation and necessitate agencies to use additional safeguards like vpn’s (no bad thing really).
Another advantage was that the PSTN was seen as strategic infrastructure with large parts of it built to carry ECN traffic intended for use after a nuclear war.
While the PSTN carried internet traffic and latterly used ip to carry phone traffic it would never have been impacted by ddos or other attacks from the internet.
The bt/Openreach infrastructure could be implemented far cheaper and far quicker, like altnets, but it’d be far less tolerant to outages.
Do it once, right and expensive, or do it cheap and expect to have to keep re doing it on a continued basis with reduced availability levels especially at scale.