Sure there is. Consider https://bugs.webkit.org/show_bug.cgi?id=36084 which is unfixed for many years now because of backwards compat issues with non-Web stuff on Mac that uses WebKit.
I can find you more examples if you'd like.
Sometimes WebKit is willing to break compat to make progress, but very often they are not. And if they were not competing with others, I fully expect them to be less willing to break compat: right now they mostly do it when the standards and other UAs force them to.