Hi, @max.c
I opened a feature request for this a few days ago: Record migration scripts in Web UI
As I see it is essentially the same as this one and the workflow I envisioned there is very similar to yours, although I had a semi-automatic script recording feature in mind for the technical solution. A fully automated and consistent sync like you described would be even better.
As for the reasons, you stated them perfectly. I agree with your post 100%
I think it is not good enough because composing models with UI is so good in DatoCMS! If it would suck more, the hard-coding option would be much attractive 