Yes, there's no internal consistency reason to not have female space marines. So what is the reason? Mumble mumble "lore" mumble. In a universe such as 40k, this is tantamount to not being a reason.
Completely disagree. I've given my reasons. You're entitled to disagree with them, but "mumble mumble" isn't much of a counter-argument.
When we get to the real reason, it's that the development of 40K lore has been driven by commercial imperatives. When they first made space marines, customers wouldn't buy the female space marines so they stopped making them, so when the lore developed later female marines were omitted. They made a load of stuff for the Squats, but they didn't sell well enough so they just cleared the drawing board by annihilating them. New aliens appeared because novelty creates sales, stops it getting stale. There is no "story" to be true to, everything is mutable as sales and PR require.
A lot of that is true, but they're not the smackdowns you think they are. To address them:
-Commercial Imperatives: True. That's academic to in-universe consistency.
-Female Space Marine Omission: Also true. However, I want to point out a difference here, in that female Space Marines, as far as I'm aware, were confined to Rogue Trader, which was an era where there was little consistency, period. Space Marine lore has developed immensely since then, and in such a manner that having Space Marines now would be, as I said, "iffy," from both a plot and thematic standpoint.
-Squats: Except the squats were brought back, and while there were real-world reasons for elimating the squats, the difference is that the setting never denied that squats, at some point, existed, and in-universe reasons were provided for their absence. In contrast, there's never been a moment in any 40K lore that I'm aware of where anyone has even mentioned female Space Marines. Squats and female Space Marines have been handled very differently, so they aren't really equivalent.
-Yes, new aliens have appeared over time, often for commercial reasons. Again, this is irrelevant to the lore, because every playable alien race has a huge amount of backstory attached to it, and the setting is designed in such a manner that there's an infinite no. of alien races that could be playable, because it's well established that there's countless alien species in the galaxy, just that we don't get to play with all of them. This isn't the same as female Space Marines which are, to borrow a phrase, "conspicuous by absence." It would actually be easier to justify a new alien species than female Space Marines because you'd have to explain why no-one's even mentioned them up to this point.
-I wouldn't go so far to say that 40K doesn't have a story, but I do agree that it's setting-driven rather than plot-driven. And you're also right, the setting is mutable. It's been 'mutated' before. I agree. What we seem to disagree on is how benign that is. I mean, for instance, if Games Workshop said today that the orks had secretly been mind controlled by some outside force all this time and were really peace-loving souls who actually detested violence, I don't think it would be out of place for me to say "well that's fucking stupid." Any setting is mutable, that doesn't mean it's good writing/worldbuilding to change/add/remove things on a whim without the proper thought attached. You can see my earlier thoughts on the necrons on what I consider a bad 'mutation,' and how it could have been handled better.
If enough women gamers decide to start up 40K and wanted a female space marine chapter, they'd just make it happen, because the story needs to respond to their customer base. (Although I guess they could satisfactorily "halfway house" and announce the Adepta Sororitas now have gene engineering so they are directly equivalent to space marines, if separate.) The sole meaningful reason there aren't female space marines is lack of customer demand / public pressure.
All true. However, my point still stands. GW could announce female Space Marines today based on customer demand. They could do anything based on customer demand. That doesn't make it a good thing to do from an artistic standpoint.
This isn't even that new to 40K. I've seen people accuse the tau of "pandering" to anime fans for instance, which is part of why some people think they're out of place. My personal views on the tau aside, if that was indeed the sole reason to do so, it's a pretty shoddy one.
Thus when you say "iffy", all you mean is from your own subjective sense of conservatism / traditionalism relating to 40K.
And your view is less subjective because...?
I mean, sure, every reason I've given why I don't think female Space Marines are a good idea is subjective. Pretty much everything written on the subject by everyone here has been subjective. But the reasons you've given basically boil down to:
1: The lore is mutable.
2: Games Workshop will adjust things based on profit.
Both of which are true, but they aren't good reasons from an artistic standpoint. You could apply the same logic to every fictional setting, but, well, if someone started requesting male Bene Gesserit or Aes Sedai for instance, my first reaction would be "go home, you're drunk."