What you say is true, but this is changing. Red Hat became valuable as a set of skills, because they developed a certification system that is provable. Those are not paper tiger certs. They are earned. While I don't personally have one, most of the people I know who work on CentOS/RHEL for a living are quite capable with other distros as well.

Debian/Ubuntu are used in surprising places, though, as you likely know. Ubuntu and Raspbian on a Pi are great at prototyping stuff. I've used Raspberry Pis for professional projects, and at my last job, a couple of Pis running Pi-hole to blacklist ads, beacons, and trackers for a public library that didn't have a lot of money in their budget for a more "proper" solution.

I don't disagree, although I think the underlying OS is becoming less important. I'm thinking about Kubernetes, which on GCloud runs on top of a modified Ubuntu. I do also see debian being used more in the embedded/low power space. But RHEL and friends seem to win in the high performance market. I also see a weirdly large amount of FreeBSD and BSD derivatives.