Game X requires iOS 8 and above. iOS 8 will only work on an iPhone 4S and above. Game X will only run on an iPhone 4S and above.
iOS is not a good example of how to do it "Properly", as Apple only let developers support OS Revisions, not hardware devisions.
For example, whilst the 3GS is no longer supported by Apple, it floated around for a
long time which caused numerous headaches for mobile developers. Why? Because it was technologically inferior to the other devices on the market, but could not be excluded by the developer because it supported the latest version of iOS.
This essentially meant that users of a 3GS would be able to download a game that simply could not run to an adequate standard. It actually became the case that Devs found a nasty work-around that prevented 3GS users from using their apps. This involved checking for the presence of a front-facing camera on the device, and only allowing the application to continue if it was found.
This in turn meant that developers were faced with two choices: "Hold back" (read: hinder) their games and applications' developments so that the 3GS users could tag along too, or face scathing reviews on the App store that most likely would affect sales. If Apple had allowed developers to restrict access to a certain
device, then none of this would have happened.
Right now, a similar problem is developing with the iPad 2 (and iPhone 4s). That device is now five years old, and is still "supported". That said, when running the latest iOS, the user experience is not great to say the least. The older tech simply can't compare to the newer devices and it puts developers in a sticky situation.
So, what's the solution here? You could have "supported" devices have much shorter, but then that means the consumer has to pay out more often in order to say relevant, or you could have a longer time-period between revisions. That second option isn't attractive if you make a profit from selling hardware, but is definitely a better option if you make your money from software.
And that is why I'm not in favour of an iterative hardware cycle for games consoles. It should stay as "defined" generations.