Yeh, sure - but two things come out of this "dominance". One: the enormous quantity of support online. Have a problem with littleknownbutcoolcms? You're stuffed. Have a problem with WP? 10 gazillion people have written about it. Then two: clients want it because it's familiar, they've used it before, they've got another site using it, etc etc.

So yeh, I take your point, but it's the peripheral stuff that makes this important IMO.