2 comments :
For me, an editor is an app similar to those already existing on Android/iOS. Existing APIs, the core REST API or the graphQL through WPGraphQL, already exist and are used to make the glu between any app (editor or other) and WordPress. So IMO, no needs ro another API… Having a new editor is “just” matter of developements without changes to the core ans no need of new API.

The second point is about blogging vs custom post type management/dedicated “metaboxes”…

With the core editor, the users could have a consistent UX between both.
And it should be the same for any “external” editor IMO.

Having the choice between différents editors could be a good thing for a blogger user profile but not sure it will be the case in a real CMS usage.