Imagine, for a second, that you get a .abc file from a friend of yours, and try to open it – just to find out that your new shiny version of ABCReader* can't open .abc files, because it is expecting .abcx files.
You'd be mad, right? You would at least expect to see a warning message, while installing the new version of ABCReader, that warns you about Backward Compatibility issues.
However, ABCReader could be Backward Compatible, so we could have liked it. We really started to enjoy using it, and would have probably continue to use it forever.
Should I start a new startup, ABCReader would be the first thing that I would install.
Note: For this post, I've made up a name for a software, called "ABCReader". If you know such a program, please let me know, so I could pick any other random 3 letters of the alphabet.