Again with the strawmen!
Have I advocated for a "nightmarish mathematical power creep"? No.
Am I asking for Mariliths getting two or three tricks up their sleeves? Yes.
You're resorting to desperately absurd arguments at this stage, Helldritch!
Strawman yourself dear Sir. My arguments are far from absurd. In fact you are the one that sounds desperate beyond measure.
Now, check the first edition marilith because in case you don't know, 5e is tailored from 1st with a bit of 3e and 4e mixed in. It is more modern and more to my taste than any other edition so far.
1st edition Marilith
Frequency: Rare
No. Appearing: 1-3 or 1-6 (in the abyss) (and yes they were meant to be encountered in numbers...)
AC: -7/-5
Move: 12"
HD: 7+7
no of att: 7
DMG: 2-8 & 6 with weapons.
Special def: Magic to hit
Magic Resistance: 80%
Psionic: 130 AE/FGH
It could even summon a demon prince (but the chances were very low).
She did have a few spells: Charm person, levitate, read magic, detect invisible, pyrotechnics, polymorph self and project image.
She had the standard resistance that all demon shared and she also had the basic spells, darkness, teleport and infravision.
Here is your first edition general. We are very far from the 2nd edition Marilith or the 3rd or the 4th. Most trick spells were useless in encounter. The only ones that were of any use were Charm person, Darkness, Detect invisible and Teleport. Project image could be of some use in some situations...
The fifth edition Marilith is closer now to her original role than it ever was in any other previous editions.
So, yes the marilith is quite ok as she is now.
Was the Marilith the power house of previous editions? No.
Did she had dozens of different tricks up her sleeves? No.
She did have a better armor class but her HP were a joke. 38hp on average.
In fifth, what she lacks in AC she has in HP. Is it a fair trade off? I think so, AC is sooooooooo easy to fix.
As for you advocation of nightmarish power creep....
The more tricks you give a monster, the more complicated it is to run. The more tricks you give monsters, the more powers will be needed by the player. Thus encouraging the return of a form of power creep.
In 5e, numerous tricks are restrained to solo lair type monsters such as beholders, demon princes and all the other that we already know (I'm not about to write all of them, no way).
The marilith was never intended to be such a type of monster in 1st edition. The fifth edition went back to the roots of D&D and modernized it thus, the marilith is just a CR 16 monster in 5e. She is not a solo/lair monster. In my opinion, the marilith is now at it right place in the MM. You might not like that fact. But it is a fact nonetheless.
And just to be clear on my standing about 5e.
I do not say that the edition is perfect.
I do not say that the edition must be defended at all cost.
I am not an apologists for 5e.
Hell there are things in 5e that I really don't like and that I had strongly voted against in the numerous surveys when it was called D&D next. Mainly the concentration mecanic on some spells. The short duration of some spells (charm person mainly) and a few others not necessarily worth mentioning as I have corrected them by myself.
All you do is asking that the marilith and some other non-lair, non-boss level monsters get more tricks up their sleeve. Well sorry sir. WotC won't do that as it would go against the philosophy of 5e. Not all monsters are lair/boss/legendary type. If you want to give the marilith some more tricks, go ahead. Do it. It is your game. But don't make it ours.