Jester David
Hero
All ability scores are useful in combat, just not for all classes. The sorcerer, bard, and paladin all get combat use out of Charisma, and being asked to make Charisma saving throws means that having it as a dump stat hurts.
Doing otherwise... well, let's look at that after this:
That makes for a very complex game.
Players have to learn the details for six different ability scores, how they related to various abilities, and then choose how to assign ability scores. That is a LOT more complicated than having one or two key scores for each class that the game can tell you to prioritize. Making characters becomes much harder. 5e was meant to be a simpler game in that regard.
It also gets unwieldy in a tabletop game. There's a LOT of RPGs out there and I can't think of one that gets as heavy into ability scores like that, with lots of optimization choices for each class. Because it's complicated and works better in a video game where the computer can manage the numbers and interactions. Video games universally tend to be more complicated in terms of character (look at all the stats and numbers in Warcraft, with Hit, armour, resistances, crit chance, etc), but much easier to play and manage as the math is done behind the scenes.
Really, most tabletop games are getting even less complicated than D&D. As rules lite as it is, D&D is still a pretty crunch system compared to the various FATE powered games out there.
It's also harder to balance. Which seems to be a big thing with you.
Perfect balance is impossible. It's an illusion. If given three ostensibly perfect choices, one will always be slightly better. Even if only situationally. Look at Rock Paper Scissors. All theoretically have the same chance of winning, yet people have won Roshambo tournaments multiple years in a row despite only having a 33% chance of winning any given game. The odds of that are astronomical. Because there's imbalance.
The more complicated the system, the greater imbalances exist. When you have a complicated system where each ability score has a different bonus in combat and every player has to prioritize and unlimited freedom to build their characters with any stat combination, that is phenomenally more complicated.
Do the math. An average 5e fighter will have a high Strength or Dex, Con as their second, the other of Dex or Str as a third and Int/Wis/Cha as wild cards. There are really two functional builds: Str>Con>Dex, Dex>Con>Str. Maybe as many as four if you put Con to tertiary. Assuming all stats have to be high and are equally valuable, that means there are 46,000 different combinations. And they all have to be balanced with each other. That's just not possible. And with a million more players looking at a game than designers, it's only a matter of time before the flaws, broken combinations, and imbalances are found. One stat or stat array emerges as just plain better. And unlike a video game, D&D cannot patch to fix content.
The balance is soooooo much harder.
