For me, its the lack of elegance, sleekness, and danger. The old marilith gives the impression of wicked grace. She's cunning, cold, a peerless tactician. She's war incarnate, and she knows it. And she will make sure every other demon in the abyss knows it, too.
2024 marilith, on the other hand, seems brutish and undignified. No jewelry or decoration to signify her rank and prestige. Not even the skulls of her enemies. Her scales are coarse, her weapons unrefined, her kills seemingly bloodless. The lack of boobs is the absolute least of her problems.
The marilith has always been a great contrast with the balor, the succubus, and the more hordey demons. At least as understood in earlier generations, balors would have been a very brutal masculine figure, while many of the other demons are monstrous beasts or basically trolls and goblins, so the marilith was a very different kind of dangerous figure. At the same time, their more regal or matronly design contrasted with the softer succubus look; even if you found them attractive, they weren't there for eye candy and kinks.
They're also just a great example of badass feminine figures with serious muscle, something that mostly died out in the 90s and is only recently making a minor comeback via She-Ra etc.
However, it's a very different era, with a very different relationship with sex, gender, and the gaze, so there are a lot of tweaks one can do. The "reptile boobs" thing is just kind of a side topic more suitable for actual reptile creatures. It's all just preference as there's not a right or wrong for a literal demon. They could literally just not have a torso and instead have floating arms - it would be more interesting than what we got!
Unfortunately this does struggle against WotC's designs of reducing demons to dumb brutes closer to 1E orcs, goblins, trolls, and ogres. The marilith being stripped of any interesting abilities hurts quite a lot. It's all very bland and homogenized, like mixing all the colors of playdough until it's just gray and purple.
I do still really like the idea of mariliths coming in a variety of forms, maybe even with one or two abilities that differ based on some of the features. Imagine a thri-kreen marilith with a gross worm butt!
