Choose your favorite metric: mindshare, running instances, development target. Free and low-end beat expensive proprietary.
"Least popular" is an odd claim. If you count Windows desktops, and then count all the Linux installations (desktop, servers, cloud, Android, other devices), I suspect Linux would prove to be most popular. Sure, add Windows servers to the contest, that's a rounding error. And what about VMs? How do you think Windows VMs stack up against Linux VMs in number?
Open source is cheap, yet its operating systems are the least-popular. But their existence is critical to a healthy market.
It's not zero sum.
What you're describing is how things become commoditized, but many companies are excellent at ensuring they aren't seen as commodities