"iOS9 is supported on a 4S, which launched at the end of 2011." As part of Apple's long running an-update-too-far policy to force users off older kit by grinding them to a halt. Having seen what iOS 7.1.2 did to my iPhone 4 and iOS 4.2 did to an iPhone 3G, well Apple aren't the best example to use.
My fear is that Microsoft are getting pressurised into doing the same thing here - releasing Windows 10 for devices that don't suit it. A more sensible path would be to keep older, low-spec devices on the reliable 8.1 release and keep it maintained in parallel for, say 2-3 years. Who knows, may be an 8.2 release with some feature updates as they did with obsolete Windows Phone 7 when 8.0 came out. Certainly better than promising the Earth!