Well the issue with the DMC menus were the CG video backgrounds. Same deal that limited the Resident Evil Remake Rerelease. Can't just extend the visible area like with realtime graphics obviously. There are of course ways to fill the screen but they include stretching or cropping.
I don't recall any instance in a cutscene in Shenmue where something really messy happened during a cutscene with the widescreen hacks. In II there is the occasional hiccup with character animations and placement but it's really no big deal at all. It's actually a lot less worrying than Shenmue even. All they have to do is change the FoW and "zoom" in the cutscenes so they fill the screen. No loss in quality and no glitches to worry about it done so. Just need to crop a tiny little amount on the sides as the black bars are actually 1.85:1 like movies and not the usual 1.78:1 style 16:9.
Please just wait and see how all the features for chosing the resolution surface instead of panicking.