Yes, but just because there is one engine, doesn't mean there is one standard. Just like during IE's monopoly there were several versions of the browser to deal with (each with their own quirks), an engine monoculture (Webkit or otherwise) does not guarantee ease of development at all.
I agree that if there is to be a rendering engine monoculture, it might as well be open source, but that in itself guarantees nothing.
WebKit can be maintained, forked etc as the world pleases.