Most certainly not. One of the purposes of the move to out-of-band signaling was to stop the various "box" attacks on the tone-based in-band signaling used previously.
They were very aware of bad actors, they just assumed that their private signaling network would remain private and thus any bad actors with access would be easy to identify and deal with. They never imagined the situation we have today with tens of thousands of telephone service providers and even large customers of those having direct access to the signaling network.
Let me try again. Built before bad actors that were the state, or someone smarter then hackers on the outside, were a thing. SS7 once you are on net with it is pretty wide open.
They were very aware of bad actors, they just assumed that their private signaling network would remain private and thus any bad actors with access would be easy to identify and deal with. They never imagined the situation we have today with tens of thousands of telephone service providers and even large customers of those having direct access to the signaling network.