Agree with you, except
The iOS 7 design WAS what made it sluggish: control center, Web-OS style multitasking, and pull-down search was what slowed it down.
Nope, they made all sorts of changes under the hood with that release that had nothing to do with animations. Yes, the animations were too much for the iPhone's A4 processor, but then again, the A4 was pretty weak with most things past iOS 4. Yes those animations are smoother on A6 and A7 based processors than A5 processors. But those animations and effects are far from the only thing changing with the OS to make devices with older CPUs run more slowly. iOS 8 adds all sorts of challenges to speed just with extensions support alone.
No, new updates usually slow down older iOS devices because: Apple's mobile processors are improving so rapidly right now. Once mobile processor improvements come less rapidly, you'll certainly see the same thing that Apple is doing to Macs: New updates don't slow them down, but many times speed them up. And older devices are supported longer.
The 2013 Apple iOS processor is 600% faster than the 2011 Apple iOS processor. The 2014 Mac intel processor is 10-20% faster than the 2012 Mac intel processor.
A. Apple making its hardware faster has nothing to do with why its software slows down version to version. It slows down version to version because they put more into it. iOS 7 is much more complex than 5 was, and more complex than 3 and 2 were, etc. That has nothing to do with processor advancement.
B. The 2014 Mac processors still use Haswell, which is only one generation up from the Ivy Bridge processors that the 2012 Macs had because Intel still hasn't come out with Broadwell chips for Apple to use (which is what should've come out in 2014; the 2014 Mac processors are still the same as the 2013 Mac processors). That's a bad analogy. Also, the technology at the low-end of Apple's minimum requirements for Mountain Lion, Mavericks, and now Yosemite, is OLD. The only reason why Apple hasn't abandoned it yet is that they haven't come up with a technology that those machines can't also run. Give them time and they eventually will, though.
On the same note, you have iOS 7 excluding iOS 6-compatible devices like the fourth generation iPod touch (which had the same speed processor as the iPhone 4) and the iPhone 3GS due to a lack of RAM, and you have iOS 8 excluding the iPhone 4 and the second generation AppleTV due to sluggish performance with the A4 processors. Given how much slower iOS 8.0 beta 5 is so far compared to even 7.0 (and mind you, beta 5 for iOS 7 was where they finally tweaked performance for release), you might be right about A5 devices running iOS 9; but the trend very strongly seems to suggest otherwise.