Upgrading is still something that difficult for some users. I haven't upgrade my blog because my poor internet connection here. I think something like instant-upgrade should be available by default. Unable to upgrade can be dangerous to your blog given that WordPress has very poor security record. WordPress just lucky because it has many plugins available. If not the codebase should be useless. I just hate someone who write upgrading is simple because overwriting is much difficult and dangerous(if you not careful) than installing itself despite maybe it will took less time.