How is this scenerio different from one where the user installed your app a year ago, and hasn't upgraded to the newest version?

You'd need to handle it in either case.

It's different because prior to this change, you could make an educated decision about whether to support an older version of your API based on usage of your app. When the legacy version reached some threshold percentage of your user base you could be fairly confident that you were inconveniencing a small and dwindling fraction of users and drop support.

With this change you can no longer be confident that app usage for legacy versions will always decrease. If your app suddenly becomes very popular (as most app developers are aiming for), you might have a substantial contingent of users who are on an old OS downloading an outdated version that you no longer support.

This was my immediate fear. We're geared up to take offline versions 1.0-1.3 of our API, which are used by less than 2% of users (and we gave them a nice banner urging them to update with a future cut-off date). This announcement has me worried, mostly from the customer support standpoint.