Not sure if Google voluntarily gave users the option. Google was forced to give users the option since Blink is a derivative of the LGPL-licensed WebKit, which is itself a derivative of the LGPL-licensed KHTML.
An argument can be made for open-sourcing the frontend of the browser. However, even the WebKit codebase has a test browser that runs on Windows (despite Safari dropping Windows support over a decade ago). This would have been present in the Blink codebase even if Google decided to keep the Chrome frontend proprietary. So people would still be able to embed "Chromium" into applications.
Isn't most of the frontend open source? Chromium looks a lot like Chrome. I would say that they would be basically indistinguishable to the average user.
Google at least gave you the option to do that.