So ok we have deduced that the amount of people who have upgraded older devices and have problems are in the minority, so what is the problem with apple allowing, this minority downgrading to an operating system that allowed their device to run properly again, maybe a built in chip allowing the device to be reset to the iOS it shipped with. Then almost everyone would be happy. I certainly would be if I could have all our iOS evinces back to their original speed, but out of six iPads and two iPhones only two of these devices work without issues.
Certainly that is a good question to ask apple; us mere mortals on macrumors can only speculate.
I think I can see why apple doesn't want their devices to downgrade. They now have to support multiple operating systems; can't leave them with gaping security holes. Developers have to worry about going backward and forward at the same time.
Then there is the perception issue. There has been a huge thread on the issues with the weather app and loss of frame rate. I couldn't duplicate it or it doesn't bother me. There have been reports how ios 7.1.2 trashed the iphone 4. My iphone 4 is eminently usable on 7.1.2. Maybe my eminently usable is totally non-workable for the next person. I don't know. Maybe people feel if their iphone on the next release does not perform exactly as the previous release, the new release is garbage when in fact the new release really works properly.