I played some games with QuickTime when I was at Apple, loved the format, even loved the stack, as creaky as it was in (gasp) 2004. The idea that the file can carry tracks that the given reader might not understand allowed me to, for instance, put codec metadata back into intermediates that would otherwise be lost, for PQ analysis purposes.

that's not entirely accurate. quicktime has been extended after MP4 container was standardized. so, the MP4 container was based on the version of QT at the time. For example, the color atom was added after that to help players with knowing how to interpret SD/HD colors. They've deprecated using the CLASP and other older features as well. So QT isn't just a locked format living along as MP4.

Some apps like MediaInfo will report an MOV with prores and pcm audio as MP4 though, so some people still seem confused.