The OP's argument seemed sort of sensible to me until I read this. Ideas need to compete. If it's not Gecko vs Webkit, it'll be Webkit vs other branch of Webkit. Monoculture will only last until "Why don't you accept my pull request, when this is obviously a better way of doing it? Because it's not better. Go fork yourself."
I think a critical aspect of open source code is that it can be forked. Wouldn't a Webkit monoculture stand in the way of this?