That's exactly how qt-webengine is packaged. And qutebrowser (based on qt-webengine) is fantastic for a volunteer effort, but memory leaks and other issues, too much stuff for one volunteer to realistically address, make it difficult to use as a daily driver. And that's just building a UI around an existing rendering engine!
The root of the issue is: W3C specifications are >100 million words, and growing at a rate of hundreds of new standards per year. So even if the Chromium code base were forked from its current state, a team of full-time developers would be required just to keep up with new W3C standards.
What I find tragic about the Mozilla mismanagement is, if one guy is willing to develop and maintain qutebrowser as a volunteer, then surely Mozilla can find developers who would be willing to work on Firefox at sub-FAANG-but-reasonable salaries without the waterfront San Francisco office.
People might praise Microsoft for moving Edge to Chromium but ultimately it's because Microsoft couldn't compete on web standards and security and didn't want to invest further resources in to something not earning them money.