(talking about supercomputers ...)
Those are highly modified custom versions of Linux.
Nope. The one I worked on (about 2 years ago) ran stock RHEL 7.1 at the time. Exactly as delivered from RedHat. With a RedHat support contract, delivered through a big computer company.
(talking about Google running Linux ...)
When asked why, one of the founders said, "Cause it's what we used in school. It was what we were used to." And no other reason.
The two founders of Google made technical decisions about 20 to 15 years ago. Since then Google has grown enormously, so several hundred thousand employees. Somewhere I read that 70K software engineers work at Google. The decision which OS to run is not made by what the two founders ran when in graduate school; I bet it is made by a committee of 20 people, who look exactly at technical and financial advantages. If running operating system "FOO" could save Google a buck or two per computer, I'm sure they would switch.
All the rest of your post I agree with.
But when I switch that off and go back to my FreeBSD workstation and server, I sense of relief from sensibility comes over me.
EXACTLY! I spend an enormous time (in my office) working on Linux machines. I am not a system administrator, so I don't have to deal with setting them up or managing them, but I still use them as programming platforms. In a previous job, I wrote kernel code and file system code for Linux. At home, I have a handful of RPIs running Linux; I even had to configure a new systemd service and administer it. It's doable. Sometimes painful, but usually just boring and productive. It's a job, and the Linux part is not the fun.
BUT: Whenever I get to administer my FreeBSD server, it "gives me joy" (to quote the TV star who helps people unclutter their lives). On *BSD, things seem logical, they are well organized, and it is easy to use. Documentation is clean and clear. It probably has a lot less functionality than other systems, but it happens to have the functionality I need. But I don't pretend that my personal taste should apply to other people. Nor that the my requirements and problems are similar to other uses. I assume that using it as a desktop machine would give very different results, but (for good reason) I don't even try that.