If you read the article, you’ll see they can update to iOS 15.
Depends which iPhone you have. Generally one has to upgrade to the latest iOS supported by their device. If Apple let people upgrade only as far as they want, there would probably be a lot fewer people on old subreleases of an iOS (for example, for those on iPhone 11 running iOS 13, their current choices are to stay there or jump to iOS 26 -- there's no way to jump to just iOS 15. Similarly no way for someone with an iPhone 12 running iOS 18.6 to jump to iOS 18.7.6 or even iOS 18.7.3 -- they have to jump to iOS 26).
Then if Apple let people downgrade, I bet a lot of people would be more willing to try a new version knowing they could go back to what worked for them. Yes downgrades are possible with in the early days of a new release but these days those versions are usually awful. When it was possible to downgrade from iOS 26 to iOS 18, it was the early days of iOS 26 which were pretty bad. iOS 26.3.1 might be great but now it's a one-way street.
These things are possible on Macs and as best I can tell it hasn't brought about the end of the world.