It gives Google a huge amount of control over the internet, like recently when they thought about getting websites to add DRM so they could only be accessed from official Chrome browsers on authorized devices and such. They also added ad tracking directly into the browser, which might not be in ungoogled chromium but it'd be more work for them to remove it, and maybe later google makes it required to use chromium. And if they get their way with the website DRM they could try to force everyone to use specifically Google Chrome and then they can track everyone. It's important that we have other browsers such as Firefox so that Google can't just do whatever they want.