[caconym_]

I think you are overstating the impracticality of mechanical standby instruments. Even glass cockpit GA aircraft typically came with fully mechanical backups until fairly recently---check out this SR22 cockpit as an example: https://commons.wikimedia.org/wiki/File:SR22TN_Perspective_C...

"Glass" standby instruments come with significant upside and not much downside, which is why they've been preferred in larger/more expensive aircraft for a while. There is nothing inherently more or less reliable about them, being fully isolated and redundant just as old-timey mechanical backups are, and they offer a much richer presentation (typically like a small PFD). However, new things are usually more expensive, which IIUC is why they were adopted first in larger, more expensive aircraft. They were considered a luxury in GA until fairly recently.