Honestly speaking, this could work if done well in Shenmue III. But I dont think it'd be cheaper, quite the opposite. Hence why people thinking 2D cutscenes might be a downgrade or cheaper are wrong.
The actual system is actually cheaper since it already exists and is already implemented.
Well, let's look it this way(Based on what I know about these stuff)
-3D cutscenes development
1.writing the way that cutscene plays out.(Alongside writing the dialogues)
2.drawing Storyboards
3.Mocap work.
4.Transfering that stuff to the game.
5.fixing all the issues that might happen.(Character models becoming buggy, facial animations messing up etc.)
6.Determining the way that the camera is going to behave in the whole cutscene.
7.Determining how character models are going to act when they are outside of camera.(Lots of games do some stuff like making a character model coming out of ground or something like that. Based on cutscene camera hacks.)
8.Doing ADR for the scene.
2D cutscene development(comic book style):
1.writing the way that cutscene plays out.
2.drawing Storyboards.
3.Making a high resolution version of the scenes and coloring them.
4.ADR.
2D cutscene development:
Similar to previous one but with one more stage of animating the whole pictures.
As you see based on what I know, making 2D cutscenes is much more cheaper and easier. Another advantage of 2D cutscenes is that you have easier time changing stuff in it.(Since the story/plot in video games always change from the beginning of development.)
It's like comparing A CGI cartoon to a 2D cartoon.(Which 2D cartoons are far more cheaper to make)
MGS 4 had a very high development costs. That was mainly due to the amount of 3D cutscenes that it had.