FWIW, totally agreed with @MayeulC's comment (to the folks who might be willing to pay for this, the content is too damn important to risk platform lock-in. For

FWIW, totally agreed with @MayeulC's comment (to the folks who might be willing to pay for this, the content is too damn important to risk platform lock-in. For this service to even get consideration, I would imagine that everything must be cleanly exportable, such that one needs only pay for continued use of the service). That said, where I would seek a tool that really adds value is auto-converting notes (text and math interspersed) into LaTeX, so that (as a value add) it can also be intelligently searched (coupled with an indexing+search solution). I think many academics/knowledge-workers might be open to paying for such a service. Bonus if you can work well off just smartphone photographs of pages.