I actually agree with your first point about me purchasing a service that could then be broken by a developer releasing an update. It's happened before and I'm sure it will happen again. Most of the time this is for an app, not an OS. I can complain to the developer and if enough people join me, it's possible that the app will be rolled back or re-updated. Sometimes the breaking change is part of a larger change to make things better. Sometimes it's a mistake. I would love a rollback feature somehow, but not sure how to implement. That does remind me of the Adobe Reader mess for WP7 a while back. They'd finally released a nice update to the PDF reader app, then somehow the version in the store showed an "update" that rolled back to the original version - losing all of the nice enhancements in the process. It was made right a couple of weeks later, but very frustrating to those of us who didn't check the version properly and just hit "update" on it. 🙁

As for the Win8 process, I see that broken cleanly into two separate updates. One goes through the "Store" process - this affects the Modern apps, including the "core" apps like Mail, Calendar, Maps, etc. They're just apps and can be uninstalled or reinstalled. The second is your standard Windows Update - OS patches, drivers, and so on. I'm pretty sure I can uninstall one of the latter if it gives me fits. I cannot revert a Store update (or at least not without some hacking beforehand to make a backup copy and hoping I know what I'm doing). I just don't see the two mixing because they serve slightly different purposes. One is mostly handled by the developers (with MS as an intermediary before publishing the new version). The other is MS directly pushing OS-level updates and the like. Office for the Surface is a little bit of an outlier here because it's a desktop app, not a Modern app so can't be updated through the Store.

I agree that patches can be released that cause damage, but if it's an OS-level update, that can still be reverted. I also agree that being able to roll back a Store update would be a "nice to have" feature, regardless of platform. I guess Android is the closest to this because all apps can come in an APK format and those can be backed up. You can then reinstall from an older APK if needed. iOS and Win8 don't support something similar to my knowledge.