It's not really a good argument. It should be about user choice. If Chrome is better than Safari, and I prefer it, I should be able to install it. OS locked down browsers is 1990s era MS anti-trust behaviour that significantly reduces overall competition.
Mobile is king - if Safari/webkit wasn't so locked down on iOS, we may have gotten even a completely different player who made a really good mobile browser. Apple's behaviour is nothing short of monopolistic and there is no good defence of it imo.