Indeed. I guarantee I could run this exact same encounter (well... using the exact same monsters anyway) against the exact same party of 6 x 10th level PCS and crush them.
The only thing I would change would be the encounter environment and the tactics used by the demons.
Weirldy Celtivan seems to be able to play (and implicitly advocate the existence of) 'highly optimised souless killing machines' for PCs; yet when DMing actual soulless killing machines in an army of demons, totally refuses to optimise them despite being in total control of the encounter placing and environment as DM.
For starters I'd probably use a 60' x 60' room. Demons are summoned into room once PCs enter. Door locks behind them trapping PC's in room. No chasme just to make it less of an absurd encounter. Roll initiative.
Vrock 1 screeches, Vrock 2 uses spores, Vrocks 3 and 4 fly to the archer, surround him and proceed to start to rip his face off. They alternate this in repeat turns (screeching, spores and 2 making attacks) until theyre all out of screeches.
Hezrou 1-3 target any 1 obvious spellcaster, surround this PC and proceed to wail on them.
Hezrou 4 uses the Help action to aid the Marilith.
The Marilith advances and makes 7 attacks all on the same PC. She starts with the tail at +9 [with advantage from the Hezrous help action]. This should hit, restraining the target (so her six follow up sword attacks get advantage). If a PC drops at any point duing these 7 attacks (all at advantage), she wails on them a few more times till they fail 3 death saves 'because demon'.
That doesn't fit my vision of the marilith leading a demonic warband that is raiding along the tunnel path in the Underdark wreaking havoc on anything she comes across. The imagery of the equivalent of a demonic barbarian horde is what I was going for. Your choice to run it doesn't fit that. Once again, you using the environment doesn't fit all things that exist in a world. Why shouldn't a demonic warband raiding the tunnels of the Underdark be a sufficient challenge for the PCs? Why do they need a 60 by 60 room? Or special tactics. Why can't they just hammer on a group of humanoids in a chaotic evil manner and go toe to toe? Doesn't that seem like something a demonic horde would do?
I'm not going to try to fix square pegs into round holes. Chaotic Evil demons are forces of chaos. When one of their generals is at their head, they should feel confident engaging a party of surface dwelling humanoids in melee combat. It really shouldn't be so easy to defeat a marilith as one sharpshooting bowman focus firing her using action surge and a bless.
Explain why you think a demonic warband acting in a savage manner trying to tear the PCs apart isn't something you could see in a fantasy world?
This whole target any obvious spellcaster again. Who is the obvious spellcaster in this six person group that has all spellcasters? I don't have a single non-spellcaster in my six man group. The sorcerer wears Elven Chain. Is he the obvious spellcaster?
Which one do you target?
Paladin/Fighter in heavy plate? Nope. Not an obvious spellcaster.
The cleric in breastplate and shield? He is casting spells. He is pretty obvious. Then when you attack him, you find out he's not a very soft target and damaging him is what he wants since your wasting attacks on a low damage dealer.
The high AC bladesinger? He sure does cast. He wears studded leather. Too had he goes into battle with 24 to 26 AC.
The plate armor wearing warlock/paladin/sorcerer? Hmm. He is casting. He does only have a 20 AC. He might be a good target.
The ranger/fighter eldritch knight Sharpshooter? He's a good one to get to. He usually stands behind the others and uses evasive tactics. Maybe you toss him in tight quarters every single battle, over and over and over again until he decides not to make an archer.
The elven chain wearing sorcerer? Hmm. There you go. He's the soft target. He is casting those fire bolts that don't do much damage. If I spend a few attacks on him, I'll really defeat this group.
So who's the obvious spellcaster? Like I told you in this discussion, my PCs don't make obvious characters. The only way that happens is if I get rid of multiclassing. My PCs have latched on to the power of multiclassing and are squeezing every ounce of power they can out of it. The party of four I made is one guy trying to optimize a party. I have six people, each one optimizing in different fashions, and using tactics they come up. Heck, I couldn't do alone what the six of them do.
Why do you think +9 is a good attack roll? You do realize it is extremely easy to get a 24 to 26 AC in this game for optimizers. Plate armor, shield, and defensive style. Take a few levels of a caster to get the shield spell to boost to 26. If you hand out magic armor, woe to you.
Right now, the bladesinger can push his AC to 31. The warlock/paladin/sorcerer can push his AC to 25 quite often. The vengeance paladin has misty step which allows quick removal from restraint. Then you have to hope the hezrou and the marilith's initiative fall right and the party hasn't unloaded on her. So many factors to account for.
Then what do you do if the party holds up in the door letting the monsters come to them one at a time? How big is your entrance? My party quite often lets the monsters come to them because they can just wear them down with range attacks.
Vrocks ripping the archers face off? That's a laugh. Why do you think they hit so easy? I really am wondering. The archer has an 18 AC and the shield spell as well. He can fire his bow in melee range. He does a ton of damage. I had the Vrock attack him. They didn't do much. They didn't hit him much. It didn't go well for them.