Allowing any random bozo to connect to the network's trusted center was a bad decision.
If the regulatory mandate to allow interconnection had also mandated the development and usage of a secure protocol for that interconnection, we'd be fine. But it mandated the opposite. Politicians got us into this mess, not programmers.
I would argue it’s the managers of the programmers who failed to foresee this as a future requirement, hence they didn’t tell the programmers to make it resilient to reasonably foreseeable changes to the operating environment.
It was not reasonably foreseeable. The Bell system had been a government-blessed monopoly since its inception. Pigs would fly before scammers were allowed to connect to raw SS7.
Allowing any random bozo to connect to the network's trusted center was a bad decision.
If the regulatory mandate to allow interconnection had also mandated the development and usage of a secure protocol for that interconnection, we'd be fine. But it mandated the opposite. Politicians got us into this mess, not programmers.