No, incorrect. there are clearly more. There are reasonably multiple alternatives.
OK, so, obviously,
(1) GNU/Linux, although it's
still a bit of a labour of love.
(2) ChromeOS
Are there any more more
general purpose destop OSs that:
(a) aren't GNU/Linux distros running slightly different permutations of the same tools - because, mostly, either the applications you need support GNU/Linux or they don't.
(b) have a market share of more than a rounding error
(c) aren't the OS equivalent of enthusiast-run steam railways (AmigaOS, RISC OS etc.)
...?
There's a difference between "this OS exists and can be fun to hack in C and Python" and "this OS is a viable alternative that supports the applications you need".
Likewise, for mobile, the choice is iOS vs Android vs. 'do you feel lucky'. The "choice" is various different skins/launchers sitting on Android (assuming they support the Play store). That's if you want your banking apps, the authenticator app you need for work, etc.
Now if your landscape is all operating systems 100% compatible with Windows, we have a monopoly. Easy-peasy.
Well, that's still the unavoidable reality for some people, although it has got better since the bad old days of the 90s/early 00s.
The sensible approach is, and has always been, pick the applications and services you need to use regularly and then buy the platform that runs them
best and
most reliably. Unfortunately, not everybody gets free choice over what applications they need - if the people paying you say you need Adobe CS or MS Office, then it's MacOS or Windows for you however much you detest Adobe or MS. If you need certain MS Office features, that narrows to just Windows.