I don't think it's enough that Chromium is open source. Yes it's very different from the MS IE monopoly, but Google has a very different business model compared Microsoft.

The value of IE to Microsoft was to lock people into Windows - both developers and users. They recognized that the web would become a software platform in its own right, and knew that if everyone ended up using web software then it'd be a lot easier to use non-Windows platforms.

Google gets much of its revenue from web advertising, so the value of Chromium to Google is to keep the web profitable for them. For example, Mozilla can't do to Google what Apple did to Facebook. They can do things like veto any move away from cookies as the primary tracking mechanism, at least until they have an alternative ready to go.

An open source license doesn't fully solve the problem because forking Chromium isn't enough. All the software on the web is built on top of APIs that Google gets a say in, and these are designed to be useful for ad/tracking networks.