I'll go even further and say with my straightest face that, realistically speaking, Iron Maiden should have started tuning down on stage as early as 1984, and that they should have recorded the majority of The X Factor and following albums, AMOLAD in particular, in Eb tuning (if not in D tuning).
This is probably their biggest flaw and musical limit. Not being too much comfortable with alternative tunings and in general with anything that is too far from their comfort zone. It's a matter of habitude, really, too bad they didn't even try.