It's just a line drawn in the sand. Nothing more nothing less. Sigil's apparently coded to open .epub files... not ePubs disguised with other file extensions. Should it also open files named *.zip -- provided there's a valid epub structure within?
I can certainly see the convenience (for you and possibly others) of your request, but I don't think it's a good idea to start catering to the whims of external programs' backup/failsafe naming conventions--regardless of how popular said program might be.
But in the end, it wouldn't really affect me in the least if Sigil was modified to allow the opening *.original_epub files, so ... whatever.