The same can be said of HTML. I realise that file archive formats are expected to be more stable (they are for archiving, yes?), but is it right to expect them to be forever frozen in amber? Especially when open source or free decompressors exist for every version of every system? ZIP compressed using LZMA is even supported in the last version of 7-zip compiled for MS-DOS.
Moving to the .zipx extension was definitely the right (and only) move. This is due to the fact that Microsoft's compressed folder code hasn't been updated in years and thus it isn't safe to send out any Zip files that uses any new features [1].