Ten years is a very long time in tech. I wouldn't be confident that the Airtag protocol will be functioning in 2035, and there are already rumors of a new Airtag and possibly a newer protocol coming up.
Add a new Airtag v2 protocol to the next iPhone and sell new Airtags only using that protocol. Why should you buy them? They could have different improvements you would like.
Start deprecating Airtag v1 in 3-4 years - and only sell new ones. There are now 3-4 iPhone generations that can handle the new version.
The next iPhone in 6-7 years doesn't support Airtags v1 anymore as it is obsolete now for many years.
Voila, they killed Airtags v1 in less than 10 years without killing the entire product line by switching to a new version. Is that unrealistic? No, thats their normal way how they deprecate stuff. It still works but only with old hardware or by not getting new updates anymore (iOS, macOS).
But this is an accessory line. Apple is pretty good at keeping accessories working for as long as possible. AirPods v1 still work. A Magic Mouse bought 10 years ago still works as long as the battery isn't dead.