I'd say the only problem with Denis is that he kept supporting something for free for so long while getting so much hate and vitriol from the community.
I'd just abandon it and find something else to do in life. If Core-js is truly that important than some company would step in to fill in the void.
And maybe I overread it, but why did he start working on core-js fulltime when there was absolutely no financial compensation in sight? It's like investing all your time in a startup that you know will never pay off.
He should have just left the project as is. The state of JS is not his fault. He is not responsible for fixing it. Once the incompatibilities become too annoying, somebody will take care of the project.
I'd just abandon it and find something else to do in life. If Core-js is truly that important than some company would step in to fill in the void.