On desktop the only browsers that use Webkit are Chrome (about 1/3 of the market), Safari (about 1/10) and now Opera (currently unreleased). So roughly two thirds of the market are on non-webkit engines which I think contradicts your argument as there's still competition there.

However, on mobile (a market which Chrome has only just entered) you could say that webkit is dominant and that we can already see problems there. To be honest though, I think that's misleading. Mobile Safari has been ridiculously dominant compared with other webkits, in mind and marketshare, and it's that monoculture causing the problems we currently see.

Serious new contenders like Chrome and Opera entering the mobile market with webkit renderers will, I think, actually help that situation to some degree by actually competing with Mobile Safari and not being half-hearted also-rans.