It could be necessary for NSA to rely on zero days or implanted bugs. Any entry point must be possible to close quickly, as soon as the enemy discovers it. Creating what you call structural back doors would make it possible for the enemy to use the same structural back door.

Along these lines, I would expect the NSA to encourage the use of cryptography and encrypted software/Secure Boot/secure communications while they ensure the NSA have a set of extra keys and can sign software at will.