I installed Yosemite on a Mac Pro from 2009 and it ran like a champ, the only thing that was slow was video transcoding. Stop with the FUD.
I installed iOS 7 on my iPhone 4 and the code is so unsuited for that device that many UI elements don't respond to touches and gestures made at normal human speed. You have to slow down by a factor of two or more for things to register. Trying to double-click the home button is an exercise in frustration.
It's definitely not FUD that apple releases OS upgrades that don't do well on older hardware.
It isn't true on PC hardware, where a modern kaby lake i5 performs similarly on desktop tasks to a a 5 year old ivy bridge i5. PC hardware has advanced tremendously in power efficiency, but much less so in performance.