Please note I said a single rendering engine, not a single browser. What's more, that rendering engine (in this case, WebKit) would be open source, which should theoretically facilitate innovation.
To me, the current situation is a little bit like train lines in Japan. Since Japanese railways are private companies, in Osaka you end up having 3-4 different companies building tracks on roughly the same itineraries, which is not efficient at all.
I believe some monopolies are necessary to enable economies of scale and increase efficiency.
An open-source monopoly might not be such a bad situation. After all even if HTML and CSS are pretty crappy languages, we're still glad we picked standards.