I appreciated this - well written and useful review of their business and why they think it didn't work.

In addition to the challenges listed here, IMO there are rapidly diminishing returns for the type of recall learning that spaced repetition enables. As you progress further in your career, there is much less emphasis on what you know, and more emphasis on how you apply it, how you communicate, and how your knowledge ends up helping others around you. I suspect most professionals decide at some point that they need to start "paging out" specific knowledge to make room for broader experience, retrieving it from the bookshelf (swap partition) when needed.

I'm also curious on the fixation with creating a startup in the VC-funded sense. Why choose able-to-find-VC-funding to be your metric of success?

For my day job I'm either "Getting things done" or Zettelkasten anyway because it's more about retrieval than memory.

But for languages, SRS is great.

And I'm also glad I memorised a whole bunch of math formulas way back. E.g. Boolean algebra I keep using an identity that I couldn't find on identity sheets by web search.