I don't think a monoculture has to be a bad thing when it's not backed by an associated monopoly. WebKit is used by Apple's Safari, Google's Chrome, and almost everybody's mobile browsers, but they all have different goals and requirements. They share code where it makes sense, but add to it their own GUIs and other innovations. Competition on the basis of performance is as healthy as ever, and the WebKit world has a lot going on in the way of new features and security.

I think that the benefits of standardization and code sharing have far outweighed the "monoculture" dangers for UNIX, and I see no reason why a similar balance between diversity and compatibility can't be achieved for web browsers.