A browser's rendering engine is the ONLY part that matters, and if anyone has a monopoly that is "bad" because competition helps innovation.
Each rendering engine winds up, for good and bad, with a small group of people deciding what features do and do not get put in. If they simply decide that feature Y doesn't get put in, and that rendering engine has a monopoly then feature Y is a non-entity as far as the web is concerned.
For example, if Google controlled the web's primary rendering engine do you think Do Not Track would be in it? NO! It took all of their major competition to add it before they did, starting with IE.