I've had a very quick look at Gutenberg and I think it shows a lot of promise. Reading through some of the comments above, I think people have to stand back and look at the popularity of visual editors for WordPress and other more sophisticated editors deployed in other products.

The "average" WordPress user probably only has exposure to one or two sites. Making the editing process easier and less frustrating for them is going to help keep WordPress growing.

There is still a text/visual view in the current beta and there looks to be plenty of scope to add new block types (inc widgets).

The only thing I didn't see was a way to have the equivalent of a page template - a bunch of pre-configured blocks laid out a certain way. I'm sure this must be on their list as no site of any size is going to be able to live without it.

The concerns about backward compatibility are valid - I would also not want to see this as the default when sites upgrade, otherwise you are inviting a support disaster. Maybe make it the default for a new install, but leave current sites on the TinyMCE until they are ready to opt-in to the new way of doing things.