Some more questions: 1) Assuming the bulk of folks use jupyter notebooks for analysis, why not integrate with jupyter or use d3.js in jupyter directly? I assume there would be a learning curve for data manipulation on js where analysts/scientists would prefer python 2) Assuming that this isn't even meant for the jupyter audience, who is it meant for within a company?
Their current target for paid customers seems to be infoviz folks at news outlets (the CTO and co-founder was at NYTimes' data viz team for a number of years): https://observablehq.com/teams
That doesn't seem like a huge market, but perhaps they're starting with this audience segment and will be branching into more DS-type features (i.e. supporting a Python kernel, trying to be a more feature-rich Jupyter).
I totally agree with you that the DS community is much much larger than the vis community. With the new funding round I think they should start doing something with DS.
Given the marketing on the page, it also looks like it's targeted at business use cases (i.e. a bunch of data scientists who want to visualize something in d3 but also want to collaborate).