There are hundreds of operating systems, multiple are in the category of "actually used by tens of millions of people": Windows, MacOS, Linux, ChromeOS, Android, iOS, webOS, Tizen, etc are the first that come to mind. iOS gets 3rd place out of those (assuming you don't count billions of virtual machines running in data centers for Linux, in which case Linux also beats out Android). iOS will also beat out Windows soon enough because computers aren't really a thing for normies anymore, most people just get by on a smartphone.

If you only consider the 3 most popular operating systems in the world to be operating systems, then sure, iOS is in last place. But maybe broaden your horizons on operating systems and you'll realize that iOS is ridiculously popular in the grand scheme of things. One day it might even reach #1 as 3rd world countries catch up economically and start buying more expensive phones, but that's just speculation of course.

Your argument is like "Athlete X was in last place in the olympics, so they're nowhere near the top of their sport", ignoring the hundreds of thousands of other athletes who didn't make it to the olympics and never will.