The overwhelming majority of development to Chromium is done by Google and not the open source contributors to the project. Maintaining a browser is not something that can be done for free as a hobby. It requires an army of full-time developers to sustain.
Given all of the major browsers except Firefox are using Chromium, the best case scenario for spinning off Chrome is that Microsoft would pick up the lion’s share of development to keep Edge up to date.
This is the same reason that all of the major Linux distributions have large foundations to support them.
The DoJ would do less harm to the internet if they just forced Google to sell off Search instead. Then they’d be an advertising and cloud services company that happens to maintain a major browser to serve their ads.
That’s fair - I should have said major browsers to be more clear. Edited above.