Chrome OS being open source is not a threat to Google for several reasons:
The usefulness of the browser is that it's a cross-platform platform. You can run the same web apps on Windows, Mac, Linux, even mobile devices.
Turning the browser itself into an OS-level platform will just create another platform that will need to compete with all existing platforms while having nothing unique to offer – after all, the same web apps run just fine on existing platforms, by their very design.
What does ChromeOS have that MacOS or Linux do not have and can't have?
You could argue that someone could build a competitor to iOS and Android using ChromeOS, but why wouldn't the same someone build a competitor using... Android? It's open source too.
Because if they take Android source without Google's blessing, they would be left without Google maps, docs, and gmail. But Google can just as easily exclude mobile browsers from accessing their web services. In today's world it would be perfectly fine for Google to only offer their services through native apps on mobile, and it's up to them which platforms they choose to publish those apps on.
Oh and then there's Widevine. You need Google's blessing to decrypt that, and even if you get it, every content provider that uses Widevine DRM can block access to their content from your browser for any reason (you can't bypass that).
How far will your rebel browser platform go without being able to play e.g. youtube? Currently they only DRM certain types of videos, but what would stop them from flipping the switch and applying DRM to every video that is monetized on youtube, and blocking your browser on the basis that it allows adblockers that effectively prevent funding of those videos?
The notion that Google is somehow – anyhow – in a weak position is utterly ridiculous. Their moat is stronger than ever.