This is incorrect. Internet connections and 3rd party integrations have changed this view of “the software doesn’t need to change once it leaves the factory”.
John Deere, Caterpillar, etc are leaning heavily into the “connected industrial equipment” world. GE engines on airplanes have updatable software and relay telemetry back to GE from flights.
The embedded world changed. You just might have missed it if your view is what shipped out before 2010.
My experience is in big scientific experiments like particle accelerators, I guess other fields are different. Still, my experience is that:
1) The control network is air gapped, any kind of direct Internet connection is very much forbidden.
2) Embedded real-time stuff usually runs on VxWorks or RTEMS, not Linux. If it is Linux, it is an specialized distro like NI Linux.
3) Anything designed in the last 15 years uses ARM. Older systems use PowerPC. Nobody has used Alpha, HPPA, SH4 or m68k in ages. So if you really want to run Debian on it, just go ahead and use Armbian.