1) What's your opinion on the apparent case that the bulk of changes come from you, as society promotes teamwork so much? Is it a case of 'small team efficency'? (Btw. I know of others' contributions too, and reliance on Rollup, ColorBrewer, matplotlib/Viridis etc.)
2) What kept you going? It's probably at least a person-year of work just on your side that's not paid for by companies, not to mention some kind of startup bet. Have you alternatively toyed with the idea of starting a value-added layer, since so many for-profit organizations benefit from your work?
3) While it changes the API, and most everything is rewritten, it's IMO a conservative upgrade in that it doesn't steer away from familiar concepts and structure. It still does DOM binding, transitions, layout etc. fairly similarly. Have you considered much more disruptive departures? As an example, React's DOM diffing is more radically different from D3 selections than the change from D3 3.* to 4.* or the Grammar of Graphics as an API concept is more radically different from D3 than the 3.* -> 4.0 API changes. Or something like moving away from the 'functional objects' concept in favor of curried, fixed-argument pure functions, perhaps prefaced with the familiar chaining API would have been a large change (these are mostly examples rather than wishlist). What were YOUR ideas, had you wanted more disruptive changes, or 'D4'?