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.