This thread began by asking about viability.
I suggest that each individual has a different definition of viability.
If one means “the device will accept the newest OS”, there’s a hard cut-off point.
If viability is synonymous with usability, your mileage may vary. I use a MacBook Pro from mid-2009 for torrenting videos. I use an iPad.mini from 2016 for watching movies in bed. I listen to streaming music from an 8 Plus. None of these gadgets runs the newest OS.
As long as I can put a new battery in my 13.mini, I’m pretty sure that I’ll resist the “new”. I’m not interested in Ai with independent agency …