But don't forget there are plenty of iPad 1 and 3GS users still using their devices and unable to upgrade to iOS 7. That chunk likely contains a fair few people unable to upgrade. iOS 7 adoption is faster than iOS 6 and 5 adoption was, according to
http://www.fiksu.com/iOS-7-iPhone-5s-5c-Usage-Tracker
Well, even looking at iOS 5 and earlier, 4% can still represent tons of people. If there are 100 million iOS users total, for example, 4% of that is 4 million--4 million people is quite a big number I would say (again, not so much in comparison to the rest of 96 million, but 4 million people is a load of people).
Plus, as there are those who can't upgrade, there are those who can't downgrade. At this point a lot of people recently got or getting new devices because any device that was able to run iOS 5 is a few years old and just on hardware specs and other features its somewhat dated in this fast moving mobile age. And the longer you use something that you use very frequently (like a mobile device) the more likely over time something can happen to it that would require you to get a new one--it breaks, the battery or some other component gets worn down, etc. So, quite a few have upgraded simply to get newer hardware, and those people, even if they wanted to stay with iOS 5, simply couldn't because the new devices don't allow for that and don't come with it. So, there's certainly that to consider in all of that as well. It's all certainly far from being as simple what percentage is running a particular iOS version, at least as far actually painting a full picture.
As for numbers of upgrades, it doesn't really mean that someone actually wanted to go to it, beyond simply seeing the update screen coming up for them and updating it without giving it much thought or even really knowing what they would be getting--which is what a typical iOS user usually does, and there are tons of these types of typical iOS users. Without a way to downgrade, anyone who upgraded, even for simplistic reasons like the update simply showed up and they did it, as they usually did with updates before, it's not really a measure of anything as far as how good or bad the update is, if people would rather downgrade, etc., etc., etc.