I have an original ipad and netflix is pretty much the only thing that still works fine on that. You can't use it to browse the web anymore because web pages have gotten so heavy it runs out of memory trying to render them. The app store is also broken to the degree of being unusable, even if you could find apps. If it wasn't so locked down it would still make an excellent linux device, but as it stands it's basically a dedicated netflix viewer.
So in my view it's a bit of both. These devices could have a longer life with manufacturer support, but the app developers are the ones driving the obsolescence.
App developer here. It is impossible to build an app for the original iPad. Apple tools will not allow you to compile backwards compatibility to iOS 6 (or is it 5?).
App developers are only a piece of this equation, though.
From (solo) developing an sms based app in 2017 there was little incentive to target an API level lower than 19 (KitKat), which provided a standard API for SMS.
The majority of devices support it so what's the incentive to support legacy software?
Now if there are breaking changes introduced to telephony in the future I'll probably maintain 4.4 support and add a check for API level. That isn't feasible for every app, though...
In case of tablets, my partner is still using her iPad 2 and Netflix has a version of their app that still runs on that.
The core software on her tablet, like the core software on my older phones that were given to family members still work fine.
Its the 3rd party apps that seem to be abandoned by software developers. That's going to happen regardless of what phone manufacturer's want.