I assume the number of people clicking and waiting are probably pretty few. Normal users probably don't realize a big update is coming until the day the notice pops on their phone. For the rest of the people, you can sideload the update easily. I don't think it is perfect, but it isn't the end of the world. The benefits of a staged roll out shouldn't be over looked either. Look at the issues Apple had with their first update to iOS 8 and having to pull it after so many already installed it. A staged roll out would have prevented that.