The problem is, that it rarely happens under normal circumstances. So you might build a logic which synchronizes your data to the server but rarely has to download it as most of the time it still has a relatively current snapshot. And the few times you have to download everything, it is ok for the user to wait a while.

But if you have to wait every time your last interaction is more than 7 days ago, the whole experience will change. And supporting a reliable offline experience will be very hard to build.