The ideal strategy for an adversary is to not implement the backdoor while you have still front-door access and people are watching. Then you get 100% deniability. You wait to implement it until you think you're about to lose front-door access.
The people in the position to scrutinize likely wouldn't be the same ones to close the door, in this instance. Network carriers aren't going to turn down support for a support contract they paid good money for unless they have a good reason.
If you put the backdoor in too soon, you get caught and you never get your devices implemented in the first place. But once you're in, the money for alternatives have already been allocated.
Because when one is discovered you use the other?
Redundancy is a hallmark of good systems.