You make it sound a little like this isn't timing and ma bell would have developed securer edge nodes, but everything on their network sophisticated enough to attack was powned by bored children.
Inband signaling for the win! Having the overhead of a separate control channel was more expensive than just doing the signaling in band.
Sure, phreakers could attack the central office equipment, and get free calls, disrupt the network some, but they couldn't make some other customer's phone explode, or melt into slag, or ring 1,000,000 numbers at once, from 5,000 miles away.