the question must then be asked, "should flight engineers be returned to the flight deck?"
And the answer that will be given is "No".
If there has to be a third fight deck crew member it makes much more sense these days to make any third bod a pilot.....they can act as in-flight relief.
In any event as Mr Magnetic has pointed out you'd still have to show that the lack of a third crewmember would have generated a less favourable outcome.