In reply to Chad.

Block themes are not being pushed as the primary experience with WP 5.9. This will be more transitional than the introduction of the block editor in 5.0. It's a far better approach that allows users to decide if they want to make the jump to such a theme.

Themes like Ona do not need to exist to use the block editor. Block themes are a separate way of building themes with the block system under the hood. By creating a standard, it allows users to customize templates via the upcoming site editor. And, the global styles system lets them modify the design. A standard also makes it easier for plugins and themes to work together without creating specialized integrations.