If you're talking about third party apps that responsibility lies with the developer. I've developed apps and found it poor form to only support the latest OS. In this particular case a huuuuuuge amount of Apple Watches support watchOS 26 so as a developer it would be suicide to kill support for that.
(especially since making money developing Watch apps is hard enough as is - but that's another story)
If you're talking about apps from Apple ... Apple can kill their own apps whenever they want.
I honestly don't agree with the entire premise that a device becomes obsolete just because it does not support the latest OS.
I’m going to be blunt, third-party developers who keep cutting support are pathetic garbage, and they’re the #1 factor that especially curtails iPhones and iPads from working.
Apple does not cut support. Older versions of their apps work perfectly. You can run Pages on iOS 9 or 10, or any of the formerly called iWork or iLife suite of apps.
And here you have the divide between developers. It’s very simple: those who are pathetic garbage, and those who aren’t. Sadly, the former win.
Let me explain: The problem isn’t baseline requirements. It’s whether they allow older app versions to function. Disney+ requires iOS or iPadOS 16 and won’t work on anything older because older versions are disabled and it forces you to update. Netflix works on iOS 10 even if it requires iPadOS 18 (!!!) for its current version. In Netflix’s case, that’s irrelevant, older versions work. Not with Disney+.
WhatsApp developers are pathetic garbage, they keep upping requirements. Telegram developers aren’t, older versions work.
Google is garbage, YouTube stops working.
Third-party developers being this appalling, pathetic, and abhorrent are the #1 impediment to device longevity. If they were a little better, you’d be able to use an iPad for as long as the popular content consumption apps worked, even if you had to restrict your usage to that.
For example, you can grab a 2018 iPad Pro on iOS 12 today, and you can use it for note-taking (apps like Notability and Pages don’t stop working), for streaming services like Netflix and Amazon Prime and Apple TV+, for YouTube on the web (for now), etc.
But some (most) third-party developers are the key impediment to longevity. This isn’t Apple’s fault.