I still have my inlaws parked on 8.1. They have a Lumia 635 and a 640. I wouldn't put them on 10. I'm keeping my eye on this as some of their apps are likely to start getting borked.
That's the same path my wife and I followed. Interestingly, I switched from Android 4.4 on an Asus Padfone X after I saw how well WP 8.1 worked on the 635 for my wife as her first smart phone after years of feature phones. I was getting fed up with the constant need to buy newer Androids just to get security updates, and the general issue of Google's snooping.
I chose WP 8.1 for my wife to use as her first smart phone after reading up on how it worked, and concluding it would be far less confusing than Android due its simple elegance, and the lack of so many duplicating apps being an advantage.
After seeing how well it did a lot of the basics (i.e. calling, texting, and the PIM functionality of Outlook for email and calendar, plus nice apps for camera, navigation with Here Drive and Waze, other useful apps such as Weather Bug, VLC, etc, I tried if for myself on her 1st 635 after I replaced the screen she shattered in an "oops moment" (got her a new 635 for $50 at a local MS mall store since she needed a phone ASAP). Then I saw a $70 deal for a 640 at the MS store, and quickly upgraded to that, and was hooked (got wife one awhile after that for $60 after she got a bit envious of my bigger/better 640 ;-} )
After WP 10 came out, I picked up a 3rd 640 on clearance at Best Buy for $30 to test that out. These were outright purchase prices for quite good hardware that would have cost a good deal more for equivalent Androids.
Over the last year or so, as I have seen how MS has progressively abandoned Windows phones overall with the half-hearted Win 10 "upgrade", along with a less user-friendly UI (for me at least - wispy fonts and tiny controls/less control), and its issues with ever fewer apps (Waze has not been updated in a long time as compared to the Android version with about 4 or 5 in the last 1-2 years, for example, and others have simply disappeared), plus the greater Google-like snooping, and the imposition of mandatory updates that render the phone unusable for an hour or so at rather unexpected times, I am not much interested at all in continuing to deal with MS and its ways WRT phones.
However, as long as the 640 hardware, and the apps my wife depends on keep on working with WP 8.1, neither of us has any intention of switching her away from it, and only if, as you noted, the apps start failing, will that be the end of her Windows phone usage of any sort, and I will have to switch her to Android as I have done - UGH for both of us then!
Meantime, WP 8.1 is not "dead" for my wife - it is simply "stabilized" ...