you can argue that maignan is never getting there, but that's then giving the ref the duty to assess a subjective thing like whether he thinks the GK has the ability to save it, which would become a disaster really quickly.

Objectively Dumfries physically prevents Maignan from diving for the ball, and thus it's offside and the correct call, whether Maignan "could/would/should" have saved it is irrelevant.