I think I understand a little better your point of view. CentOS became so important for the HPC community that most software is now validated against it. So even if RHEL itself were to become free (as in beer), people won't switch to it (or at least be reluctant).
My all personal systems are Debian, however when I install something research related, it's always CentOS. There's no question. I even manage a couple of research servers at my former university. They're CentOS as well.
Moreover service (web, git, documentation, etc.) servers are CentOS too to keep systems uniform even if there's no requirement. So it powers the whole ecosystem, not the compute foundation. That's a big iceberg.