[Fradow]

> I've got several still functional 4.x-5.x devices [...] that can't even use the market anymore

4.4.x and 5.x can certainly use the Play Store. I use both of those versions regularly, and they still works well (in fact, I keep 4.4.x support, and do not plan on stopping soon).

In Europe, a few manufacturers still sell 5.x devices. In particular, my manufacturer of choice only recently decided to finally upgrade the small tablet / low cost option to give us the chance of having an OS that's not 5 years old on purchase /sarcasm.

May I mention that contrary to iPads, older devices allow to side-load older app versions (that you can easily find online). I recently re-used a 4.0.2 device and installed VLC and Firefox on it (all it will ever need). And for 4.2+, you can just use F-Droid, most apps are going to be compatible.