We've seen how this ends because mathematicians have been sharing Mathematica notebooks forever. It's not pretty.

Like you I see the appeal, but they're a usability nightmare beyond a few lines. Part of the problem, I think, is that you can't really incrementally improve them. Who wants to refactor a notebook and deal with all the cell dependency breakage?

So they start off okay and then slowly become terrible until they're either irreplaceable or too terrible to work with and a new one is started.