"Possible under canon" does not truly include the image at the beginning of this thread, unless you are giving a lot of weight to marilith's shape changing abilities (which have varied across editions), and suggesting that the marilith is able to use those to take on the form of a generic looking six armed monster. In the same way, you could literally portray the marilith as a two armed dwarf cleric as 'possible under canon'. Possible perhaps, but not helpful.
Throughout the discussion, there has been a lot of switching between 'demons' and 'mariliths', in that things that apply to demons apply to mariliths by virtue of them being demons. The concept of demons having gender in the same way that humans do is neither supported nor undermined by editions previous to 5e. Certainly some demons adhere closer to human(oid) characteristics than others. I also feel that when we're talking about editions previous to 5e, that any 'gender' discussion is probably more akin to 'sex' in today's language. When 1e described mariliths as female, it very much meant physical breasts rather than a concept of self that was feminine. Demons don't have (canon) gender roles in their society, but different types of demon do have roles expected of them. Mariliths are strategists, as of 2e. (However almost zero canon encounters of mariliths exemplify a strategist rather than a six armed buzzsaw killer, Night Below's comes to mind as an exception.) So some demons have a defined gender, some demons exist in human genders and some don't have any. (Probably. You can investigate the alkalith on your own.)
As such, I agree with the poster who said that the change is likely due to an inclusivity push, but I disagree that it was well done. A manhole was something that had no actual gender, sex or any 'man-like' qualities. Whether you agree with them becoming referred to as utility holes or not, surely the lack of humanity in a manhole is not up for debate. A very inhuman object was given a name that described its function and the older name has fallen somewhat out of use. The marilith on the other hand, did have female qualities, and different tables likely had different takes on how far beyond the torso those female qualities went. Personally, I would have enjoyed seeing an elf or dwarven torso-ed marilith, since those are often mentioned and not often drawn. Even an orc would work given that they are being folded into the humanoid player races. A marilith with a female orc torso would be recognizable as a marilith in a way that the poorly drawn snake blob monster is not. And if the art direction really wanted to challenge the conventional take on the demon as female, it ought to have provided a well drawn male or intersex torso. (You can use 2e Geryon as a model for the male if you need one.) Running away from the humanoid form is not 'more inclusive'; it doesn't include anyone that the previous version didn't. No one playing the game is a generic snake blob.
tl;dr You can like the generic snake blob, but you cannot say it isn't changing the previous lore. I don't like the generic snake blob (both on its artistic quality and the direction) but I'm open to marilith lore changes/evolutions.
