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.
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.
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...