Yes. Has nothing to do with planned obsolescence. iOS 9 has a lot more going on than iOS 7. Background processes allowing Siri Intelligence, widgets, machine learning, etc. We want iOS to keep pushing forward and at the same time we’re surprised when 3 year old hardware doesn’t run it just as good.
Apple is, actually, really good with longevity. My old 6S came with iOS 9 and worked great. Years later, it’s on iOS 11 and the performance is still good. It will be very capable for 3 years. With next iOS it will probably start to lag, but still work. That’s 4 years of it being usable.
While I agree Apple isn’t truly guilty of planned obsolescence via new iOS versions, the user should have more choice in whether to update their phones. Unfortunately, security updates are introduced along with new software versions, so exploits discovered now will only be patched for those running iOS 11. However, those wishing to stay on older software to avoid device slow down will not be able to maintain security without updating the OS. This is problematic.