In of the videos of Google I/O, developers were talking about the "bus factor". It basically means if one of your developers were to get run over by a bus, how would the development process of your product be affected? That's why they encouraged documenting the work done etc.

Same principal applies to your current situation too I guess. If that 1 critical developer is run over by a bus / decides to quit / has a health problem / has a baby etc things may stagnate more than you can imagine.

A key advantage of a startup is to be flexible and lean, which means you'll be able to adopt and discard things faster than the "big boys".

So basically rewriting the core to let everyone (especially newcomer employees) get productive on it easily would be a better idea.