[rixed]

You want to be able to change the container while making sure you do not alter the contained stream.

I've always thought it would be simpler if we used different files for the stream and the meta data, but that's probably just because I never looked more closely into it.

[bathtub365]

From this perspective it may, but now you have multiple files you need to keep track of and it’s not clear if one is missing depending on the underlying stream structure (i.e. multiple audio or video streams instead of just 1 of each)