In regard to the comments above and without any doubt a dev will have to define block usage by utilizing the user roles.

The coming FSE features mentioned above are certainly interesting and I am looking forward to explore them fully.

To be honest they sound much more interesting for a theme dev like me than anything else block-based yet (except for the widget areas, which we kind of got a preview of already). The reason might be that I am coming from a dinosaur world where you keep your content separated from your design -- as much as you can at least. Fancy block patterns will work on landing pages, but probably not on ten year old blogs and magazines trying to keep their content future proof for the next 20+ years. Nobody wants to back edit years of content for a new screen-size, design fad, a simple theme change or whatever might come in the following years (kiss - keep it simple stupid!). Looking at past experiences, I just say short-codes and translation plugins!

While this might at least partly explain the hesitance of theme developers jumping on the "block wagon" fully committed, I fully share Justin's outlook that there has never been a better time to be excited about theme development than now.