maybe no amount of assurances from Microsoft would reassure you, but if you're in charge of a hypothetical multi-year, multi-billion dollar R&D programme and you need a desktop OS to run your software on, who would you allow to reassure you? Apple? The Debian security team? A few hundred specialist developers you just hired to build you something from scratch on top of FreeBSD?
I'd do some sort of analysis to tell me which was best, and then I'd trust the best for which I could get the source.