I don't see the argument. Trademark has nothing to do with money. They don't lose their trademark if they give away something gratis. Canonical doesn't lose their trademarks just because you can get Ubuntu without paying them. Before RHEL, there was Red Hat Linux which was free to use.

That is so very incorrect. Way back in the beginning, they offered RHEL without support but with updates. At some point they started charging for access and eventually they raised the prices high enough that I couldn't afford to use it any more.

Google is free to use, that doesn't mean that just anybody can take their trademark.

Trademarks have nothing to do with whether something is traded for money or not.

Red Hat bought out CentOS now. They own CentOS. There would be no trademark dillution from a legal standpoint if they used their own trademark for their free version of the distro.