No. You have what you believe and are making arrogant, asinine assumptions about those that disagree with you.
From what I see my games work just fine, I challenge my players whether it's the experienced longtime group or the newb group. I use what they have given us and either toss out stock monsters and encounters for one group or beefed up monsters and encounters for the other. It's really not that difficult.
And yet the game designers don't design adventures with this in mind.
You're not struggling to find a reason. You're just not paying attention to the modules being produced by the company you claim follows a certain baseline.
I don't play many APs but have played a bit, as others have pointed out it's not a strict adherence but they generally give you a handful of detailed encounters then some random encounters for you to throw in as needed. It's up to you as the DM to ultilize their detailed and random encounters and manage your game appropriately.
The monsters are lacking because they are lacking. It is not a debate, but a provable fact. A party of adventures designed using the baseline you claim the game follows (which it doesn't) have far more options to crush creatures than the creatures have to crush them. I could design a base party using the base rules and easily defeat a balor or marilith. They are very weak.
Well if you are throwing 1 Marilith or Balor at a full party(let alone a party of experienced, optimized, magic item equip'd characters), even if you were actually using the encounter guidlines, yes it will be weak. But if you don't just toss a Marilith at the party and say have at it, but actually design a Marilith or Balor encounter it's a different story. Add some henchmen with some varying abilities, add cover/terrain that helps you use tactics, then actually play those monsters tactically, you will find that they aren't so weak. The fact is, you are running a game for optimizers, giving them magic items, and not following the base guidelines so the monsters are weak. You have created the atmosphere that makes them weak and design/run it poorly which makes it even weaker. These issues are not the designers faults, they are yours.
How did the marilith take better advantage of terrain than your players? How did their teleport help them? It takes an action and eliminates their ability to attack. Did they seriously not scout ahead or use missile fire which utterly defeats a marilith? Did you realize their parry doesn't work against missile weapons? Did your players play dumb and wander up to melee it? Perhaps your players aren't good at tactics or building characters? Might that be a reason your players had a hard time? It could be. Whereas tactically intelligent players that build ranged monsters may just rip your encounters apart?
Teleport can very effective in moving around the battlefield in and out of cover, if you actually design the battlefield. With a handful of henchmen to keep portions of the party occupied, to box in a ranged characters and set up situations where the Marilith can set up to charge in to a boxed in ranged and grapple them, they then can beat the crap out of the ranged characters. I generally include several different types of henchmen, sometimes a caster or abilities I can use to control the battlefield to a degree, sometimes a healer, sometimes a few ranged monsters to cause issues. In the end it doesn't take much to go from Party focuses and decimates the Marilith, to the party is struggling and having to blow their big abilities to win. My players in my main group play super tactically, so I play my monsters super tactically to match.
Is that not what some of us are trying to do? Find a way to make it work for our play-style. I think that is the point of the discussion. To point out that 5E out of the box isn't well-designed for experienced, tactically oriented optimizers. It has weak points in monster and adventure design that make it too easy a game. We are pointing out the specific problems. What custom options are a problem, what spells, what class options, and the like.
That may be what you are trying to do, and many people have already given good advice on how to do it that you seem to ignore. If you wanna run less encounters per day, increase the number of monsters in your encounters, choose them wisely and play them tactically to eat resources. If you wanna give your players magic items, increase the monsters HP to account for it. If you wanna throw a single boss fight at your party, give your boss henchmen, legendary resists and actions, and buff up their HP to give them more survivability and more action economy to bring them more in line with the party. If you have a group of optimized players, optimize your monsters and optimize your encounters to match. If you don't like to have 6-8 encounters between long rests, use the rest variant rules to extend rests out to fit in more of the encounters you do want to have. None of it is very hard, it does take a little time but that's what a DM signs up for.
I'm sorry if you can't see the problems with mariliths and balors. I've run them as well against optimized and normal (mean rules as intended) parties, both times it got smoked. My parties tend to control terrain. Only if I do something like Flamestrike did where I make the environment something they can't alter due to power levels outside their control can I change this. The first game we played we had some challenges. As soon as the group found out Smiting and Ranged attacks were number one for offense, every single party they make now is comprised of those types of characters with a bard and some kind of healer support. 5E monsters can't handle optimized parties any better than 3E. So heavy modification is required.
My party always wins, sometimes they plan and come up with tactics that are very good and when they do I generally let them it and sometimes they get an easy win. But more often than not they work for every victory. Just from what you said, I suggest you add a few other demons in with the Marilith or Balor. "Mariliths are often encountered as captains at the head of a demonic horde," is straight out of the monster manual. So select a handful of lesser demons for the Marilith to captain. Since your party likes to control the battlefield, maybe toss in a Barlgura or 2 that have access to entangle and phantasmal force, or maybe Chasme with their drone ability, Vrocks, Shadow demons. All of these have some fun abilities, you put a few of them together with a Marilith and now they have some battlefield control ability, and your party HAS to use battlefield control and deal with more than just focus firing a Marilith. These are all simple things that don't take massive effort as it's just stock monsters from the book, you are just using several together with varying abilities which brings your encounter more in line with what you players can do. Still in the end you have to use those abilities and play as tactically as your players but I guarantee that if you do that, your players won't just smoke them. Past all that, if your players have lots of magic items, I would also consider buffing up the HP of whatever you toss into the encounter just to help offset the increased power your players have from those items. and if your players are REALLY good, I might also consider some legendary actions on the Marilith, specially if it's your 1 encounter that day, other than that you just have to assess your party and figure out if it's really necessary. But that's just my 2cp. My group's Marilith encounter was very fun for me and them.
These threads tend to have a bunch of folks like you thinking we're saying, "5E so bad." While someone like you says, "5E is not bad. You're just dumb." When what's really being discussed is, "What are the optimization options in 5E. How do they affect the game. How do we get them under control. How do we design monsters and challenges for optimized parties because like 3E, 5E can't handle optimizers very well."
The thing is, if they change the system to handle optimizers as the base solution, it will no longer work right for the new players. What I am saying is, the game has everything you need to make it challenging for a party of optimizer. Does it take some time? Sure. 5e is a very resource dependent game and optimized or not, resources still have limits. Increasing the encounters per day will be a good step to reducing your labor, but it's also not the silver bullet depending on your group. Aside from that, add monsters, use strategy, add legendary actions, add Lair actions, and know what your creatures can do before the game starts. You wanna DM for optimizer, then you have to be an optimizer too. Being an experienced DM/player, you should be able to do that where a group of new people likely cannot.
