Would be awesome with another update! 
Are you still looking at mystnb <-> ipynb, or have other ideas cropped up?
Fortunately, I’ve found this topic a bit late.
I want to register that I’ve deployed another format for notebooks that we use inside my (relatively small) team (which seems to undermine spirit of pessimism here).
There is a number of restrictions in format: python-only, designed for small teams, ignores most of jupyter metadata;
And there are big pros: diff/merge/keep all (even large) outputs in cvs/ability to maintain along with changing codebase that notebooks use (important point not mentioned in thread at all)/ability to use IDE for checking or editing code in notebooks.
ipynb is not restricted in the team, but discouraged for storing in git.
So far it has been ~6weeks since migration, so let’s see if it “survives summer”. I have reasons to believe it will.