WebKit can be maintained, forked etc as the world pleases.
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.
Even today, mobile users typically account for only about 10% to 15% of a site's traffic, for sites that don't directly target mobile users. Mobile just isn't as important or widespread as a lot of people think it is.
Their February 2013 results are at: http://gs.statcounter.com/#mobile_vs_desktop-ww-monthly-2013...
It gives a good sense of the current situation. Mobile browser usage is under 15%. It's nothing compared to the usage of desktop browsers. The growth from 2011 and 2012 isn't even that impressive, given the all of the hype and attention surrounding mobile.
Don't forget that on mobile platforms, not including Windows Phone, WebKit is already the standard engine. Safari, Android's stock browser and Chrome all use WebKit and developing for mobile is a ghetto, especially because each version has their own quirks and bugs (and there are dozens of Android stock browser variations in active use at any given time).