The problem is that there's no guarantee the people who want to do "us" harm will use the compromised security tools, and in the past (i.e. during the Cold War, arguably the greatest existential threat to the US since 1865), the NSA seems to have recognized this. But really, you're missing a crucial point.
It doesn't matter if the NSA and DoD have secret encryption systems that are even proof against quantum computing. Even attacking purely private sector systems, including defense contractors, telcos, power companies, banks, and think tanks has the potential to cause crippling damage to the American economy. Our defense forces rely on that economy, from soldier pay to logistic supply chains to fuel to military hardware. Our high tech military could be crippled if enough damage could be wrought on financial, information and communication systems within the country, and every weakened box, every backdoored security protocol, is a potential attack vector against that.
We already know that China and North Korea have state-funded hacker units going after government and industrial secrets; they build their own Linux systems so they won't be susceptible to the NSA's usual bag of tricks. If they repurposed those units from intelligence gather to electronic sabotage, that's bad for everyone the NSA is supposed to protect, from the President and Joint Chiefs down to little Timmy in Kansas City. Maybe you think the risk of random, illiterate terrorists destroying the country is so much greater that you're willing to run that risk to capture every nutcase the FBI can entrap. I certainly don't think so, and neither do most non-NSA cryptographers.
What now, yourself. Tau is boring.
If it has syntax, it isn't user friendly.