Yeah, having such low end devices at one end of the spectrum would push Apple to make iOS 9 efficient, thus, greatly improving performance on all devices. However, because they have so many devices to test/develop for, it can complicate things.
My friend has a Galaxy S5 and the new lollipop update already uses around 1300MB out of 1.6GB (not sure why it said 1.6GB available when the device has 2GB of RAM?) I remember my Galaxy S4 had usually around 800MB of RAM being used on KitKat.
So, what one can take from this is since Samsung is growing so damn fast spec wise, it freaking ruins older devices and general OS quality. Her S5 was already stuttering a good amount, worse than my iPhone 6 and my mini 2. However, the mini 2 has much more heavy stuttering in some spots compared to the S5. Even so, the S5 had a consistent stutter over just about everything it felt like. They don't have to worry about how bloated their version of android is because they just keep chucking power at it. It just does not seem sustainable. At all.
Maybe she had a stuck process or something, I don't know. It was just generally stuttery... My experience.
That my friend, is called TouchWiz... Samsung's horribly inefficient resource hogging UI over the top of Android. I owned a Galaxy S III for a couple years in '12-'13 and it was pretty annoying. Then I flashed a stock AOSP based ROM, and it was smooth as butter.
If you've used the HTC One M8 or the Moto X it is an entirely different experience on the same basic hardware. Amazing how Samsung can mess up things so much. It's gotten better at least with the newer version coming on the S6. But that doesn't give them an excuse to keep using the older versions on their "older" phones. But they probably will.
Anyway, I'd love to see iOS 9 on my 4s if it's an improvement. It's not crucial, but it is my backup phone. I just went and replaced the glass/lcd/home button and back glass with new panels and it looks brand new. I just love the design of it and I plan on keeping it around.
More importantly though is the iPad 2. We have quite a few at work that are used as POS machines, while it wouldn't "need" iOS 9, security updates and better performance would be welcomed. Some were even made in 2014 before they discontinued them.
I think at least some A5 devices will se iOS 9. iPad mini, iPod touch, and Apple TV are still being shipped and sold, and are A5 devices. Though the Apple TV is a bit different, and single core, it's still an A5. Gives us some hope.
Time will tell