That's a strawman. Microsoft already had a browser engine, no one's talking about writing one from scratch. They are unable to maintain one.
> For starters, even if they fork, they can still keep cherry-picking fixes and new features from Google's Blink.
Yes. All of them. Because otherwise they aren't compatible with the Google-controlled web anymore. That kind of "fork" is pointless.
Microsoft had to choose between adding their own enhancements and standards (the reason for having an engine into the first place) and playing catch up to Google. With Blink they can have both.