License will cause many problems

[lbmn]

A huge fraction of projects will NEVER use a format with a restrictive license like GPLv3!

Please choose a license from http://copyfree.org/standard/licenses

[jpakkane]

This is research code just to prove a point. If there ever is a proper release it will have a different license.

[chr4]

GNU tar uses GPLv3, too.

[lbmn]

tar has multiple implementations, including BSD tar. It's not new technology.

License restrictions mean not everyone can use a new format (including for purely philosophical reasons, as with me and other copyfree.org fans), so it's a lot less likely to gain popularity - unless someone writes a Copyfree implementation...