When they stop releasing security patches for that OS version 2 years later, it becomes more risky to connect the thing to a network. Or take in any data from the outside, really, whether it's via Bluetooth, or USB drive.

And then there's 3rd party software that will stop supporting that old OS version, in part because Apple's dev tools make that difficult.

Eventually, Apple's own services will stop supporting that OS - no convenient iCloud support.

Finally, the root CA certs bundled with the OS will become too out of date to use.

I'm planning on putting Linux on my Intel Mac Mini soon. But when a M3+ Mini goes out of support, will we have that option?