I say the following as a firefox+ubo user:
There are many useful things that can only be implemented for Chromium: things like the filesystem API mentioned in this post, the USB devices API used to implement various microcontroller flashing tools, etc. Users can have multiple browsers installed, and I often use Chromium as essentially a sandboxed program runtime.
SOME users can have multiple browsers installed. Some can absolutely not. In fact, 1.6 billion users can only have one installed and it's not Chrome or Chromium based.