A pattern is copied, not referenced, and that is the trade

Inserting a pattern copies its markup into the post, so changing the pattern afterwards changes nothing that already exists.

pattern          copied on insert. edits are per post.
                 changing the registration affects only NEW uses.

reusable block   referenced. stored as a wp_block post.
                 editing it changes every post that uses it.

template         applied to NEW posts of a type. not retroactive.

the question that decides: should an editor be able to
diverge from this, or must every instance stay identical?

A pattern is a starting point and a reusable block is a component, and choosing the wrong one produces either a design nobody can update or a design nobody can adapt. The copy semantics are what make patterns safe to change — a registration that changes cannot break existing content, which is the opposite of a block’s save output. Explaining that distinction to an editor once saves several confused support requests.