That doesn't help if someone breaks into the endpoint or a client machine, compromises it and captures your credentials. The system is weak at the point where you decrypt the data. In the case of the Clinton campaign hack, it appears they got access to the DNC voter analytics data, possibly the famous system a couple of Sanders IT team gained access to. If they had stolen database credentials, they could have exfiltrated the data regardless of whether it was encrypted at rest.
Using a proprietary operating system would make that intrusion more difficult, correct?