Denzark, I reckon, purely as a popularity-with-the-fans point of view, it'd be so HUGE a step to retcon such a thing that it would honestly appear to be as if they were acquiescing to "political correctness" or similar perception of social dilemma..
I suppose the most obvious point is that to rock the boat in such a way would not please everyone, and that is something they'd not be willing to do, considering that tabletop 40k itself is over 50% of the value of the entire company and all its divisions..
With any other bit of the background, changes can sometimes be subtle, sometimes more than subtle, but never something so drastic.
C'tan changing from being actual gods to broken amnesiac controlled shards of gods? I'm good with that.
T'au Ethereals being created by the Eldar as controls to use in the fight against Chaos? Cool by me.
Heck, look at the Squats.
Spontaneous ULTRA RACE DEATH BY TYRANIDS RAARRRRGGGHHHHHH!
That was throwing a match at a very flammable pile of fanboy rage, it's the 40k equivalent of Greedo shooting first..
The fact they've been subtly slipped back into the fluff as Demiurg and now in the 6th ed BRB? I'm fine with that too.
I reckon it'd simply come down to lots of hardcore fans doing the OMGZ ITZ NOT RITE WHY IS THERE LADY MARINEZ I HAET DIS thing and getting worked up over nothing.
Why risk the fallout when you've done pretty well so far?
Plus, in a purely sensible argument, I'd say just look at the models.
Astartes are hulking slabs of muscle so utterly different from even human bodybuilding champions that if you did the same to a girl, she'd surely come out looking the same, wear the same armour, and be indistinguishable from a male Astartes once suited up.
Even GW wouldn't sculpt curvy boob-and-butt plates for female Astartes.. Would they?