>absolutely essential OS tools could reside, if necessary, on a very small file system usable for all rescue scenarios
Am I stupid to think you can mount these small, stable, reliable filesytems as subdirectories within the /usr folder? I'd really like to join in the bikeshedding here, but the everything-is-a-file metaphor has a complementary everything-is-mounted-somewhere concept which leads me to think 'meh'.