Reimplementing it would not be an option. Making WebKit the reference implementation would give Google and Apple such an enormous advantage in development cycles that any browser that didn't use it would be unable to compete. You'd be expelling everyone but Google and Apple from the W3C.
If WebKit becomes the standard, every user from now until forever will be using the same pile of C++ to browse. That may be fine to you; I personally don't think that WebKit is so good as to make it the gold standard for the Web. Imagine if that had happened with NCSA Mosaic...