First, Chromium is also based on WebKit so that means really only one browser engine. Second, I imagine so many web sites and web applications have, knowingly

First, Chromium is also based on WebKit so that means really only one browser engine. Second, I imagine so many web sites and web applications have, knowingly or unknowingly, made themselves dependent on WebKit or Chromium specific behavior, it's almost impossible to write a new browser compatible with all (or even most) of the web.