We've seen what happens when almost everyone uses one browser engine (Internet Explorer): that vendor gets undue influence over the direction of the web, and their proprietary extensions make it difficult for others to compete.

We would end up in the same place if everything is based on Blink/WebKit; arguably we're partway there already.

Hopefully Servo will provide a competitive browser engine and full browser in the future as well, with the bonus of being great for embedding. But it's not quite there yet. And other potential competitors have their own problems. Firefox is, for the moment, the most popular and only viable browser not based on Blink/WebKit.