Even if Linux was a monopoly that would not necessarily be the case for ever, as the example of the Windows monopoly demonstrates. But Linux isn't even a monopoly today.
People absolutely will sell you Linux, and they make a lot of money in the process.
I'm not even sure I'd describe it as a monoculture either, if it was that would probably have accelerated its growth even more. Imagine where Linux would be if it were like FreeBSD in having only one distro, one libc, one set of system libs.
You're starting to see it as a server backend lingua franca but even there, there are multitudes. And there remain a great number of organizations that run on one of the BSDs, Windows, old mainframe OS, and so on.
Even if Linux was a monopoly that would not necessarily be the case for ever, as the example of the Windows monopoly demonstrates. But Linux isn't even a monopoly today.