In addition to what the other comments are saying, it's not at all clear that maintaining a suitable fork of Chromium would be less work than maintaining a comp

In addition to what the other comments are saying, it's not at all clear that maintaining a suitable fork of Chromium would be less work than maintaining a completely different browser implementation. Google is clearly trying to drive the web into an unsavoury direction so the divergence burden of such a fork might increase with time. Also, it's never a bad idea to implement the same thing twice, from scratch. Working with an existing codebase necessarily puts you into a certain mindset with its own trade-offs and constraints where moving in a certain direction might be much harder than if you are starting from scratch. Would we ever see innovations that Servo brought forth if all we had were Chromium forks? It's hard to say, but it's definitely not obvious.