NOBUS is only NOBUS until a spy gets their hands on the escrow master key (or until Donald Trump shares it at a dinner party on a lark, for that matter). If RSA's signing keys can be compromised¹, anything can be compromised.
[1]: "The Full Story of the Stunning RSA Hack Can Finally Be Told," https://www.wired.com/story/the-full-story-of-the-stunning-r...