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?
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).
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).