It seems like you're implying they'd be too good to ever get caught, but... they got caught. The trouble is, making a backdoor less obvious makes it more likely that if they try it 10 times they don't get caught all 10 times, more likely it gets into production before they get caught, more likely that it stays in production for a year instead of a month, etc.
I mean, didn't the NSA also get caught by Snowden? They intended it to be a secret.
But the Juniper hackers are the NOBUS failure because changing the locks on a backdoor that somebody else had installed is easier than getting one installed yourself.
I don't think you're following. "NOBUS" doesn't mean "nobody but us can ever find out about the backdoor"; it means "nobody but us can actually use the backdoor". Ironically, the Juniper PKRNG backdoor --- I assume it was Chinese --- is also a NOBUS backdoor!
