I started to write a reply here, but it became a blog post instead. Two excerpts:

After they contributed a giant patch, we switched the schema over to a custom post type and taxonomy, rewrote a lot of the code to conform to our coding standards, and started to build out an API. After a round of usability testing, we made some numerous design and UI changes. We also needed no-JavaScript support and RTL support, and our new UI called for new JavaScript, so most of everything ended up being rewritten.

Point is, that’s how it works. It’s called iteration.

It was a very sizable contribution that then drove our stalled development in the direction we were grasping at for some time. But it was also just the first of hundreds of iterations (like any other major feature) -- the patch was an early prototype of a feature we wanted to scale to millions.

More here.