> make a core lib with all the nuts and bolts which devs build clients around

That is modern Anki. The core is a Rust library, which all the clients (desktop, web, Android and iOS) use. [0]

[0]: https://news.ycombinator.com/item?id=46299897

I learned something today, thank you.

Even so, I believe there's room for another open competitor or two in this space.