Huh? You do know in essence your both your use cases are the same.
Not sure what you're saying. What I am saying is that Apple achieved iOS 9 on the older devices through a new way of creating an iOS devices where by they started with a core version of iOS (With the new APIs and Security updates) and enabled new features one by one to achieve the best performance mix (in theory).
What I'm saying is that is how they can very possibly do the same with iOS 10, in creating that core version of iOS 10 possibly just with any new security updates that iOS 10 brings and little else, to ensure that newer devices aren't held back by the old ones and that the older devices aren't bogged down further by features they can't handle.