After I left the Finnish church (in Helsinki) some 15 years ago, nobody came to see me or anything. I have had the occasional Jehova's Witness ring my doorbell, but no-one from the state church.
The point is not that the GPL should enable end users to make easy modifications to things like firmware - the GPL doesn't say so anywhere, and it may not even be possible.
The GPL, version 3, does require this. See section 6: "If you convey an object code work under this section in, or with, or specifically for use in, a User Product, [...] the Corresponding Source conveyed under this section must be accompanied by the Installation Information. But this requirement does not apply if neither you nor any third party retains the ability to install modified object code on the User Product (for example, the work has been installed in ROM)."
"Installation Information" is defined to include "any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source".
But then you get a phone call every time they do want to install something. Better is to install sysresccd on a USB hard drive, configure the system just that once the way the users want it and use partimage to store an image of the computer's hard drive on the USB drive. When they get to trouble, restore the image and you've got a working system again - you could even set up a script to do this. In case of security updates, you of course need to restore the image, do the installation and create a new image, but this is much easier than hunting down all the spyware and whatever garbage the users have installed on the computer.
"there appears to be no compelling technical or business case for upgrading to these new Microsoft software products. Furthermore, there appears to be specific reasons not to upgrade."
Save the whales. Collect the whole set.