I think the biggest issue would be the audit. Recall that NT started out as part of OS/2 which is owned by IBM. I suspect that Edge/IE has similar issues, because it was evolved from NCSA Mosaic it probably has legal issues preventing it from being open sourced.
I know for a fact that the ZIP component of the windows shell would be an issue as that is definitely licensed to Microsoft instead of being owned by.
Open what they can and keep binary blobs of stuff they can’t?
(Never going to happen, not just for philosophical reasons, but I'm sure legal reasons too)