When were you planning to upgrade your three year old phone? I upgraded for same reason as you (6 plus), but when I found out I could have paid $85 for a new battery to restore much of my performance my reaction was, meh. I would have still upgraded anyways.
Even if there wasn't CPU throttling to keep old phones usable, software marches on adding features and benefits.
I have always been amazed at Apple's ability to shoehorn iOS versions and new features into such limited memory capacities (compared to Android for example). But there is always a limit to what can be done.
My daughter's iPhone 5 shipped with iOS 6, and is currently running iOS 9. Is it slower? Yes. Does it have more features than when released, heck yea.