Smart enough? Really? So are all the people playing a more complex game like Pathfinder/3E or GURPS just smart enough to understand the rules, but not smart enough to modify 5E? Because you know, modifying 5E takes such a high level of intelligence? I don't think criticisms of 5E have anything to do with the intelligence of the persons making the criticisms or their lack of willingness to modify the game. I think some of us making criticisms just don't like the new paradigm like the three to five round by design combats. They feel unsatisfying and anticlimactic. I don't call you unintelligent and uninspired because you enjoy such things, I would hope you would refrain from calling people with legitimate criticisms about the 5E game system unintelligent.
Sorry but I am struggling to find other explanations. The OP posts up this post as his anecdotal proof that the game is broken, but in his example he admits it was a single encounter and not 1 of 6-8 a day. He acknowledges that his player's characters are built, outfitted, and played above the base assumptions. He picks some fairly powerful creatures then plays them horribly, he put no thought into using any type of tactics or designing his encounter with even simple terrain and strategy, and did no prep to know what his monsters could do, then whines about how the system is broken.
The system is designed for the lowest common denominator. Inexperienced players, 6-8 encounters, low/no magic. I think this was a good choice because it targets new players and as a system is much easier to manipulate from that baseline upwards to accommodate various ranges of experience and play-styles than it is to start at a higher level and then have new players trying to tweak the system downward. So you have this system set at the base, then you give your players a bunch of magic, run 1-2 encounters an adventure day, and fail to design your encounters to ultilize terrain, startegy, and the creatures abilties effectively and then complain that the system is at fault. You complain that monsters like Marilith's and Balor's don't have enough abilities but they are lacking because of your game design and play of them. If your characters are walking into a fight with a single Marilith as there 1 to 2 encounters per day and are all decked out with Magic, then yes the Marilith is woefully outmatched. But let's think about it, Truesight (a strong ability), resistance to non-magic weapons (strong if your players don't all have magic weapons like the game is designed), 7 attacks per round, one of which is a tail attack that grapples the target which can be the first attack granting advantage for all the rest of the attacks that round (very strong ability), can Parry as a reaction to increase their AC to 23 for an attack and is reactionary so can do them once every single turn in a round, magic resistance, Teleport, and solid saves. I have used this monster in an encounter with a couple lesser henchmen and it was no cakewalk for my group. I adequately used it abilities, I designed the encounter in a way where I could the terrain and it's abilities optimally for best results.
So maybe you can better explain exactly what the issue is cause I am just not seeing it. I guess I do see it, you want to play the game completely different than it is built and in that case, yes it is broken for YOU, but that's your fault not the system's. If you have a Ferrari and then try to use it as a dumptruck, don't go whining that Ferrari's are junk cause you can't haul dirt with it. Either play the game as it is designed or understand that you, as the DM, will have to tweak it for your style of play and you, as DM, will need to play your encounters to a level that is consistent with your players. And again, they have given you all the tools to do this, they just aren't doing it for you because they want the game accessible to new/returning inexperienced players.
