1. The game's difficulty, environmental (such as hallways) is balanced around it, changing that now is just plain dumb tbh. It would delay the development by a long time.
2. Even having 4 characters gets crowded in many places, when you need to jumo over your characters in narrow areas to get to the enemy. 6 characters would eb nightmare.
3. Cycling through characters in combat takes even longer, beng detrimental to game's combat speed whihc is already on the slow side. Especially when you have one character go alone ins neak mode, it's really annoying already to press end turn to skip the turn on the remaining three every turn, Doing it five times would be plain hell.
4. As stated before, the need to specific roles are much less needed in D&D so you can play without a healer etc (I do).
5. Replayability increases by a lot because even after you finish the game there will be a couple more new fresh companions to play with ij your next setup, with their unique storylines to explore.
6. It's more challening, which means more fun.
I used to miss 6 characters as well, but now because the game's so fun and enjouable even after 6+ playthroughs, I prefer 4 man. I will replay this game many times, no need to have all companions with your the first time, and it adds more variety.
I am sure a mod with add 6 man party for you to enjoy anyway.
We've discussed many of these before. I'll only touch on a few. You are certainly entitled to your opinion, and I'm not suggesting that there is a right or wrong answer here. However, I have many reasons for liking party of 6, so I will defend it and try to point some things out about your reasoning.
1. Difficulty is based on Larian's homebrew gimmicks and rules and monsters. Proper 5e rules, monsters, etc. would change the entire difficulty in a hot minute. Encounters that are currently challenging, as you say, would become deadly if they actually used the right rules and stats. So, a party of 6 would be required to NOT have an OP encounter in this game if they used the proper 5e stats and rules. Also, I've played party of 6 mod, and it works just fine as far as environment, hallways, etc. Would it delay the development? Not really that much. Change some numbers and such. If a simple mod can do it, they can do it. Change Party Max Size to 6 instead of 4, like the mod does. The hardest part would be to implement 5e rules and stats, but they need to do difficulty settings for the game anyway. So, provide us with a Core Rules difficulty, party of 6, and we're good.
2. Jumping is only an issue because Larian is still needing to work on the whole mechanic of it. They require you to jump because they don't let you pass through other character's spaces. An enemy should block you. A friend should not. They should allow your own party members to pass totally through each others' spaces. So, if someone is blocking you, it should only be an enemy. THEN Jumping like a superhero wouldn't be necessary just to maneuver around the map. You wouldn't NEED to feel crowded by 6 characters or 4 in tight spaces because they wouldn't get in your way as much. This CAN and HAS been done in games like Solasta.
3. I've touched on this before. 6 party members would not slow down combat as much as people think. In fact, many times it would speed it up. Imagine teaming up on a single ogre with a party of 6 as opposed to a party of 4. 6+ attacks in a single round as opposed to just 4-ish. One ogre could be potentially taken out per round if 6 members gang up on it, each taking 1-10 HP off of it. Meanwhile, it may take multiple rounds to try to take out the same ogre with only a party of 4 especially if it gets to go at some point and knocks out one of your 4 before they get to attack. Sure, the skipping thing is totally true. If stealthing or doing something similar, if you have to just skip the turns of everyone else, that can get a bit more annoying than only a party of 4, but how much slower are we really talking? Especially as they are speeding up combat more and more, I'm thinking the end result probably won't be THAT painful. But, of course, that's a matter of opinion.
4. TRUE D&D REQUIRES class distinctions. This is something else we're fighting for. You SHOULD require a healer. By the Nine! The fact that you don't is just sad. Larian has severely crushed the classes, making them so they really aren't unique or needed at all. Imagine playing the game and not being able to throw potions to heal. Imagine not being able to use every scroll no matter which character you are. Imagine a potion being an Action to use. Imagine that when a party member falls, you have to make a Medicine roll to revive them before they die. If you fail, they continue to roll to see if they die. Imagine that you don't have magic items that give you the ability to use spells that your class wouldn't normally be able to use. You see, part of the problem is that Larian ISN'T being true to D&D, and that's why you don't need all your bases covered in your party. You don't need a rogue because everyone can pick locks, disarm traps, use potions as Bonus Actions, etc. All these things are NOT D&D.
5. With the Party of 6 mod, the characters interact MORE with one another when all traveling together. Things you normally would not have happen actually happen. They talk to one another and have banter and everything SO much more than with party of 4. This is, of course, another matter of opinion, but I much more prefer the party feeling like one cohesive group actually talking to each other and bantering than having some sitting at camp doing nothing because a party of 4 is, for some reason, "full up." Never mind you have a tadpole in your head and you're all in the same boat. Wyll and Astarion'll just sit back and do nothing while 4 people try to take on a goblin camp of 30 or more, some of which are ogres.
6. Challenging, again, is based on difficulty settings and the fact that they don't use proper stats. Trust me when I say, this whole game could be so much more fun if party of 6 with proper stats. Very challenging still and lots of fun. I played it out via tabletop. It works and encounters are still rather deadly at times. The big difference is, there's more balance with proper stats and rules without people jumping like superheroes, shoving people 30 feet through the air, spiders spitting poison and teleporting like mages, imps with actual resistance, intellect devourers that use devour intellect and try to body thief you, mud mephits that don't multiply like a virus, wood woads that act like wood woads, drow with sun sensitivity who only fight at night because the sun hinders them, goblins who don't have thunder arrows that knock you into explosive barrels and blow you and your whole party to kingdom come...
Ahem. I digress. The point is, this game COULD be truly balanced and awesome with party of 6 fairly easily. They just need to implement a Core Rules difficulty and set the monster stats to what they should be using more appropriate monster tactics and abilities.