Actually people don't expect it to be updated in most cases. In fact, Apple is generally forcing their hands with constant nagging to update and the psychologically taxing red notification dot that people do not know how to get rid of.

Most people would be just fine buying a phone as it is and using it as is for the rest of it's useful life. But they can't, because Apple came with a clever marketing trick to make things easier for them: "free" updates for everyone. This way they get to keep working only on one OS version, deprecated stuff aggressively and largely no need to care about security patching stuff after a few years.

If you are in the "ecosystem" they will force you to upgrade your OSs to be in sync if you dare use one of their apps since they are tied to OS release (dumbest things ever, but of course it's on purpose).