This all seems like exactly the behaviour you would expect if having D3 users is not profitable and having Observable users is profitable. This would explain why the Observable team's explanations don't entirely make sense: the real explanation is "we are making things worse for you because we hope to make money by doing so". It would explain why they're unmoved by users' complaints that something allegedly meant to make things easier for them is making them harder: the real goal is profit, not making things easier.
On the other hand, (1) using Observable is free (at present, at least), and (2) in other respects mbostock has always seemed genuinely interested in providing good things for everyone to use. So I hope the cynical view in the previous paragraph is wrong. But it does seem like it explains what's happening better than "we just want to make things better for everyone, and Observable is so awesome we can't help using it for everything" which is pretty much the official line.