> The problem with long term support for my Android phones has actually not been the fact that Android devices have incredibly short security update windows. That issue has been somewhat mitigated with the newer Google Pixel phones which have five years of security updates

5 years of updates is still a short time. We only have one planet...

> The biggest issue for long term cell phone support is, even if we get an OS with a 10-year security update timeline like Rocky Linux, will the phone itself be able to make calls on whatever cellular networks exist 10 years from now?

I don't think this is the biggest issue : operators usually maintain a certain type of carrier for at least 20-30 years (in France, we are only talking about shutting down 2G - which still raises a lot of issue because of the many IoT devices using GSM...)

The biggest issues are IMO

  - lack of parts to repair old phones
  - no possibility to manage bootloader keys / relock the bootloader (not even mentioning devices with locked bootloaders)
  - "stable-api-nonsense" ideology and no BIOS/UEFI/ACPI for smartphone => no way to have "one firmware to rule them all"