As per discussion with @mtias here and via DM, we should merge the Comments Query Loop block into the existing Post Comments block, with the following behavior:

• If it doesn’t have any child blocks: Use the current Post Comments behavior, i.e. render the “monolithic” WP comment_form(). In the editor, we’ll display the placeholder.
• If it has children: Use the Comments Query Loop behavior (i.e. render those child blocks).

If it’s newly inserted: insert child blocks as specified in its default template (as Comments Query Loop does already).

This means that the block will provide backward-compat for Post Comments (where it’s been used for existing themes); OTOH, it will provide the new functionality by default when newly inserted.

Quoting Matías:

The important things:

• Replacing the block not creating a new one so we keep the namespace.
• That existing uses of the PHP comments don’t break.
• That inserting the block for the first time in a template uses the new set of blocks.

Note that there will be some edge cases that can lead to slightly awkward user experience, e.g. trying to edit a previously inserted empty Post Comments block (will show a placeholder in the editor). Matías said not to worry about these for 6.0.

The rest can be improved through iterations [after WP 6.0]
Including a “convert to blocks” and the addition of some nicely designed patterns

cc/ @WordPress/frontend-dx @priethor @gziolo