In reply to Bill.

If anything, you will eventually have more control over locking down design and layout elements than what is possible with traditional WordPress. You would be able to lock templates, enable or disable blocks and patterns based on permissions, and more. Much of this is possible now, and we can expect more advanced features in the future.

Remember that "locking down" things like this outside of the basic role capabilities is not native to the classic editor either. Developers have to build such things on top of it, regardless of the editor.

As for developing from code instead of a UI, I don't see anything having to change in that regard for agencies and other developers.