The fact that there hasn’t been a second security update for iOS 15 doesn’t mean another one isn’t coming which it almost certainly is.
unless you're on the latest major version you're pretty much out of luck
As I just mentioned, iOS 12, a 4-year old operating system got updated a couple of month ago and iOS 15 will continue getting updates obviously.
To put things in perspective, updating iOS 12 means devices as old as the iPhone 5s were updated. The iPhone 5s first shipped September 2013—over 9 years ago!
I suspect Android devices released 9 years ago haven’t been updated in a long long time.
Besides, by the summer of 2023, something like 80+% of the installed base will be on iOS 16, which is what happens every year. iOS 16 is already a little ahead of where iOS 15 installs were at the same time last year [1].
[1]: https://appleinsider.com/articles/22/09/14/ios-16-adoption-s...