Mozilla could contribute to Blink, but that would not help it prevent Blink from adding e.g. Web Environment Integrity ("DRM for the web"), or other additions that are user-hostile. Being in control of an engine gives way more sway over that engine.
This is why browser diversity is where the action happens. Firefox and Brave can just nix the DRM components. Similar to how Brave itself or Ungoogled Chromium cut down Chromium's telemetry components. One engine underneath, many interfaces and options for power users and the concerned. We all save on the extremely hard work of building an entire other browser engine, and then expecting the whole industry to support it.
