”we don’t like x, we are going to leave WP”

Do not worry. As some people will leave, many others will join. We care only about the net migration of users.

As a web developer on projects using php frameworks, wordpress, drupal etc., Guttenberg is something that I just imagined would be perfect even before it's first commit.

For many of us, developers, it will be a central point where using a common api, we will be able to offer easier customisations, custom features and offer easier help teaching our clients/collaborators on how to actually edit a page to obtain that exact output they want.

Plus, for all other visual builders, I found no issues in starting a new post/page and deciding which to be used. Also, already created ones default to whichever editor was used initially, so current data would not be compromised on future updates.

I think the haters against it, just have too much free time to comment about it and no true knowledge of how the project has truly evolved from a developer's point of view.