I honestly don't see an issue with the mpeg-4 example.

Regarding the potential for fragmentation of the png ecosystem the alternative is a new file format which has all the same support issues. Every time you author something you make a choice between legacy support and using new features.

From a developer perspective, adding support for a new compression type is likely to be much easier than implementing logic for an entirely new format. It's also less surface area for bugs. In terms of libraries, support added to a dependency propagates to all consumers with zero additional effort. Meanwhile adding a new library for a new format is linear effort with respect to the number of programs.