What if your backend APIs have changed and you never expected to support that older version? I can see a lot of the older apps not working at all in that case.
You'd need to handle it in either case.
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app developers are aiming for), you might have a substantial contingent of users who are on an old OS downloading an outdated version that you no longer support.