Can someone explain to me why the Gutenberg team doesn't want to have a "Classic" button in the corner of the screen that restores the Edit Screen to the way it is in WP 4.9?

Then we get the best of both worlds - all the old plugins still work, developers can update on their own schedule, new plugins can be built for Gutenberg, and users will get the easy Gutenberg writing experience when they open up the edit post screen.

Am I missing something? I haven't followed the project closely.