> Red Hat made linux mainstream.
1. In the English-speaking world, mainly meaning North America.
For instance, SUSE is just as old, and brought Linux to the mainstream in Mitteleuropa. Not just in "DACH" (the German speaking world: Deutschland, Austria, Confœderatio Helvetica), but also for instance it was the first Czech-language distro. I worked for SUSE in Prague for 4 years, and it is still big in that part of the world.
Conectiva brought it to LatAm. TurboLinux to Japan. Etc., etc.
2. FSVO "mainstream" -- Red Hat Linux (note, not RHEL) was not really very useful as a desktop OS. I ran it, I used it, but trying to install something as complex as a desktop using raw RPM with no dependency management was nightmarish. That's where Mandrake came from: Red Hat refused to bundle KDE because Qt wasn't 100% GPL, so Mandrake ported KDE to RHL and bundled them.
Caldera was even earlier, and OpenLinux 1.0 with KDE was the first desktop distro I tried that was polished enough to make my primary OS for a while.
RHL was mainly a server OS, and as such, not very mainstream for most people.