The only example (barely) where open source wouldn't benefit security is if proprietary code was yours and you knew exactly what it does, period.

You're not talking about security, you're talking about trust. A problem of computing yes, but not one which open source would eliminate. As long as you're not a professional security engineer and fully audit every software including a libre-version of the Linux kernel yourself before you use it on open-source hardware, you have to trust other people. You have to trust all contributors to open source software that they neither try to place anything malicious intentionally nor introduce vulnerabilities, you have to trust the developers of a project to do a good enough job in reviewing contributions, you have to trust the package maintainer that he build the binaries from the actual source, he has to trust his compiler and the compiler which compiled his compiler, you have to trust the repositories that they aren't compromised, you have to trust the hardware that it does what you expect etc.

And all this mainly assumes protection against intentionally introduced security flaws because you can absolutely trust nobody to do a good enough job to never overlook any vulnerability. You're hopefully aware that there are serious security flaws and 0-days in open source software.

By all means, elaborate.

What's not clear about my statement above? You can't take things which might be established in the future from one OS and compare them to the status quo of another OS, and then come to a conclusion which is exactly the opposite from the real situation right now. The common Linux desktop how it's used by 95% of the people right now and how it would be used by most Windows users if they switched right now, has absolutely nothing to do with everything running in containers or using Wayland. There hardly is a single distro-desktop combination where Wayland is really usable out of the box for everybody. Even Fedora 24 with Gnome which is sort of leading in the process won't have it as default.

Kernel sandboxing features that Chrome is utilizing.

I'm confused. Are you saying that Chrome is more secure on Linux because it's sandboxing is better on Linux than on Windows, or are you saying that the Kernel uses sandboxing mechanisms for all processes, similar like Chrome uses sandboxing.