Re: Why would they?
Microsoft's decision to stick with the name Windows 10 when pushing out "feature updates" indicates that it is moving towards Apple's policy. It's probably only due to the large enterprise customer base that it will continue to provide support for older versions.
While I can understand Apple's general approach, I have occasionally been extremely annoyed by their insistence on fixing some bugs only in new versions: a bug in Bluetooh in Lion was particularly annoying and AFAIK never fixed. There are always good reasons for not upgrading immediately to the latest MacOS as evinced by the frequency of subsequent patches.