If you can't evaluate it, you can't trust it. Plus, Apple had gotofail, and MS has had its share of issues as well.
In other words, with proprietary sw, at least SOMEBODY evaluated it and placed their seal/name on it. With open source, you are relying on a hope that somebody out there somewhere does it. And in various cases, we've seen how that turned out.
You're not making a substantial argument, but if someone has a track record of writing completely safe software that doesn't have to be secured through third party products, like Microsoft, I will be willing to listen.