If it's going to slow down the phone, Apple should simply take that model off the list of iPhones that should upgrade. My 4S is super slow now. This isn't a pretend issue. It's real.
Exactly. I don't understand why Apple doesn't just exclude the 4S from the list of devices compatible with iOS 9. All iPhones before that are excluded. Apple always has to decide where to draw the line. Just say the 4S is no longer supported through new OS upgrades. How did they decide the 4 was no longer compatible? There must have been testing done or at least some HW analysis.
https://www.apple.com/ios/whats-new/#compatibility
I have a 3rd gen iPod touch and a 2008 Macbook that both run perfectly with the most recent allowable OS upgrades. I can't upgrade any further than that, and recognize that they are no longer supported, and I'm perfectly fine with that.
It is perfectly reasonable to expect your devices to continue functioning normally. Obviously, HW will eventually fail, but there should not be an artificial failure due to SW that has no business running on that HW.