> It's easier maintaining a fork of Blink than an entirely different engine. Google being the primary maintainer doesn't mean that browsers can't make large forks.

Maintaining a fork is not fun as taking patches from the origin repo becomes more complicated the longer the fork exists and the bigger the divergence becomes.

> How is having N engine developers implement some new addition to the EMCAScript or CSS standard for each of their engines not duplicated work?

It would be duplicated only if they all maintained, say, a Blink fork.