A good thing for consumers, but I wonder how developers feel about this. If they fix some bug that may have been causing big problems on a server-side component, won't they now have to worry about these older versions being used?
Sure you can. Netflix will sometimes have to do this. The old software just isn't allowed to talk to the server anymore.
I don't recommend that unless you have a mission critical bug, and even then I'd discourage it. A user is in charge of his/her device. I version APIs on the backend and then simply push out updates like any other software. If they choose not to use it then not much I can do. Eventually I'll turn the older API versions off and they'll either have to upgrade or stop using the app.