Thanks everyone! Manjaro rocks, in no small part thanks to you the community! I should have watched the wiki more carefully and especially appreciate those who pointed things out with patience. 
I do want to keep recommending Manjaro as a primary system for family, friends, even kids who are just starting to learn Linux. If their boxes fall over due to proprietary planned obsolescence, well, it’s not entirely their fault. I don’t expect magic from Manjaro devs, I’m a dev myself and fully appreciate what goes into the project. Just trying to help Manjaro succeed by putting fences around the pitfalls of user experience.
The hard work of compatibility consideration was already done; the warnings were written in release notes1. But plenty of users were predictably broken by the upgrade, and it could have been avoided simply by encoding the compatibility knowledge into the upgrade process. Something like Melc’s suggestion: if (gpu.is_kepler() || ...) { warn_and_explain_with_link_to_wiki(); offer_driver_selection_change(); }.
Many tenacious types (self included) will figure things out and fix their systems. But some users will simply leave and never come back. The troubleshoot post offering clear descriptions of issues & fixes is great, but most users aren’t going to read everything, so coding a few safeguard lines would be tremendously helpful for them. Even a hard stop in the user-friendly interface would be better than steam rolling boot capability: if (gpu_driver_is_mainline_proprietary() && gpu.is_known_unsupported()) { refuse_to_upgrade_with_link_to_explanation(); }
Guard rails are better than signs saying, “One more step and you’ll fall to your doom!”
Sure, some of us will drop into a terminal to run commands with forcing options, but then we’re the type that expects breakage. Not so for the kids; they just need the machine to come up so they can learn how to rice their desktops with Linux. Gotta start somewhere and I’d like it to be Manjaro!
Thanks again for everyone who took the time to consider and read all this. Cheers!