By "like native does" do you mean that native exposes its internal decoders somehow?
To my knowledge, it does not.
It's just the most obvious design. Microsoft isn't bound by the limitations of the GStreamer API, they have no reason to recreate the demuxer.
It'd be of uncompressed data
You mean compressed, right?
and the fact that one GStreamer pipeline is easier to debug than two
And three is even harder. Demuxer, video decoder, audio decoder. (Though any specific issue is unlikely to impact more than two.)