Similar deal with WebKit. At ~15% marketshare between mobile and desktop followed with Gecko at ~5%, it's the only real holdout against Chromium domination. If browser engines were opened up on iOS, you can bet anything that Google is going to go bananas with marketing Chromium-based Chrome for iOS and web devs will happily back those efforts, pushing most or all of that 15% over into Google's lap. At that point Mozilla will have an even more difficult time holding on as an increasing number of devs only develop against Chromium.