There are sometimes new features which don't work well on older hardware for obvious reasons and are therefore excluded, but I think it's a stretch to make out that older hardware doesn't get any benefit from newer software. People often point out Siri as the prime example of a feature that was released on the iPhone 4S but "held back" from the iPhone 4, but forget that the 4S hardware was heavily optimised for Siri in ways that the 4 was not - with a revised proximity sensor for raise to speak, noise cancelling technology built into the SOC, dual core processor etc. Yes the 4 could be hacked to run Siri but it could never run it very well, and that's not the experience Apple are aiming for.
More recently it's difficult to think of too many examples where new versions of IOS have held back features from older hardware apart from those instances where clearly the older hardware didn't support it - like camera slo-mo etc. That's nothing to do with software really and everything to do with the capabilities of the hardware. A similar thing happened with older macs which don't have the Bluetooth 4.0 needed for handoff - but my 2009 MacBook Pro can still pick up calls and send texts via continuity now, something I never expected it to do pre Yosemite - and all of this is via a 2012 iPhone 5, not this year's model.
I get why people are resistant to new versions of iOS on their older devices. There is an established tradition that each year, there is one generation of devices (this year it's the iPhone 4S, iPad 2 etc) which are likely on their last release of iOS, and which don't run perfectly on the first .0 release. Typically they get better on the .1 release and there is some more improvement from there. It's a pattern we've seen repeated for the last few years - last year the iPhone 4 was practically killed off by iOS 7.0 before being rescued by a later point release. The point is they do fix these things - it's just a shame they don't optimise them well for these older devices at the start.
