The argument against that is that you are hopelessly insecure if your security strategy is to hope no one develops tools around vulnerabilities in proprietary s

The argument against that is that you are hopelessly insecure if your security strategy is to hope no one develops tools around vulnerabilities in proprietary systems. That is going to happen and they are going to leak whether the NSA does it or not. The best bet is to aim to build systems like seL4 and use properly configured well engineered OSS solutions until solutions like seL4 become available.