Maintaining a browser fork is a lot of work. And Google can simply decide to close-source their browser engine once their takeover is complete. What could anyone do about it? Google just keeps adding features until all Chromium forks die off.
The problem is not the source but the way Google can affect the standard.
The biggest is DRM. If stuff like that gets into the standard where only large players can make a compliant implementation, then it becomes a huge problem for the web.
