>As an example, in Jan 2026, Apple published iOS 12.5.8 which provides updates for iPhone 5s which released in Sept 2013. That's 12.5 years ago. The equivalent would be to connect to the internet using ADSL in Jan 2000 with your IBM PS/2 rocking in intel 8086, 512 kb of RAM and expecting an update for your DOS operating system.
The updates for ios 12 are all security updates, not feature updates, so your comparison to "connect to the internet using ADSL in Jan 2000 with your IBM PS/2 rocking in intel 8086" doesn't really make sense. The phones stuck on ios 15 are basically unusable because many apps don't support it anymore. At best you can download an older version from a few years ago, but that depends on whether the backend servers were updated. Apps that insist you use the latest version (eg. banking/finance apps) basically unusable.
A phone is not unusable because some banking apps don't work on it. It didn't even ship with said apps installed.
Believe it or not, "apps" are an important "feature" of a smartphone, even if it's not theoretically bundled with it. Moreover it's not just banking apps, those are just the first ones to go, but any that don't keep backend compatibility will eventually break.