> RedHat moved into that space by providing businesses the same model of support
But it didn't, not back then. In the 1990s, and for most of the company's first decade in business, it offered a free distribution and made most of its money from selling merchandise. Which is to say, not very much money at all.
It's only around 2003 but its management hit upon the idea of having a premium enterprise distribution, and making the only way to obtain it being to buy commercial support for it. That was a stroke of genius, and over the last 20 years, it's made the company very rich indeed — in stark contrast to its first decade.
> while ALSO having a desktop version/free version for people to learn on.
Again, that split only came after about a decade in business. For its first 10 years or so, there was only the free version.
Also, more to the point, back when Solaris and HP UX were big, Red Hat was still very small and mainly in the business of selling T-shirts and mouse mats.
It's a bit of a generalisation, but to an extent, it was Microsoft Windows which put commercial UNIX out of business, followed by Apple macOS which showed that UNIX on the desktop could be as good or better than Windows.
Windows established a marketplace of COTS high-end x86 kit with decent graphics capabilities and capable storage subsystems. Apple successfully piggybacked on that to make desirable easy to use RISC UNIX workstations for consumers, using their own processors but otherwise pretty much off the shelf PC industry parts.
And those things put together are what created a marketplace in which Linux could thrive and compete effectively.
When Linux was first invented, a high end PC had a crappy VGA card on the ISA bus, and many of them didn't even have mice, sound or networking as standard. I was there. It was grim.