Who is to judge what is reasonable?
I think the only objectively reasonable expectation is for the device to continue its offline functionality for as long as the hardware works (eg. playing movie files, taking/viewing pictures, typing offline notes, offline games, etc), because it requires no additional work or support from anyone. But to keep your device connected to the world, ie. any online functionality, requires someone somewhere to put in the work to keep your device compatible because the world keeps moving. Of course there is an expectation that online functionality continues for some period of time after purchase. And that period of support is built into the price of the phone, app, etc. But how much of the purchase price goes toward support and how long that support should last, is pretty hard to determine objectively. If users continually paid developers specifically to keep their device supported, that would be a different story. One could expect indefinite support then. But my guess is it would get more and more difficult to support over time and there would be diminishing returns, and probably therefore no market for that.