That's wild speculation, and the elephant in the room (Linux), while a different beast, is a pretty strong counterexample for how being 'just' open source oesn't doom you to being slow or insecure.
> It's crazy to assume that if it were an open source project it would somehow get the core right faster or not expand on its ambitions.
I don't think it's crazy at all. A (say, community funded) purely open source project has no distractions and has nothing to do but to get the core right. It also has no incentive or reason to expand its ambitions.