I agree with you to a large extent--apps that get an overhaul in terms of UI can be a huge problem if the changes are poorly executed. Yahoo's been doing this kind of thing over the past 2-3 years, possibly in an attempt to emulate Google's services (and therefore retain those customers who might want to defect to Google)(remember that Marissa Mayer was originally from Google). They changed their webmail site to look and behave much more like Gmail, but it was glitchy and clunky, and probably drove more people away.
The problem with making multiple versions available to the users, though, is that it starts to cause the dreaded issue of fragmentation. If different users are using different versions of the app, then it's harder to push out new features to everyone. Security is probably also a consideration as well, since older versions may not be as secure as newer ones. It'd be nice if there were a way to toggle between the new UI and the old, but that would also add to the size of the app, which would be bad for those people with low internal storage.
Another thing to keep in mind is that the purpose of the update may have been to add more Material Design language to the app, because users also complain about apps that aren't consistent with the rest of Google's design language.