> Can't you improve a compression algorithm and still produce a still valid decompression input? PNG is based on zip, there's certainly ways to improve zip without breaking backwards compatibility.

That's just changing an implementation detail of the encoder, and you don't need spec changes for that e.g. there are PNG compressors which support zopfli for extra gains on the DEFLATE (at a non-insignificant cost). This is transparent to the client as the output is still just a DEFLATE stream.