Isn't the fact that you can do the entire old games alone while it is designed for 6 a proove that the balance is not that bad ? (Even if, of course it increase the difficulty).
We'll also have difficulty level not to increase too much the difficulty. I.E P:K have tons of options so players can create their own custom difficulty level. That could easily solve the problem of "a game designed for" and allow players to custom their experience.
It's probably comes down to what one wants from an RPG - for me the fact that players breeze through BG1&2 with a single character is a sign of the system being fundamentally broken. It is afterall a party RPG with a class system designed to limited what each character can individually do. If single character can do it all, IMO it breaks what the game is about. And to counter argument: "what the problem if you choose how to play it?" - in BG1&2 I usually find 2 or 3 characters being constantly useful, while others (like spellcasters) usually hang back and do little to nothing, and then contribute only to the most useful encounters. That's not something I felt though in modern RPGs (be it PoEs, D:OS1&2 or Kingmaker). I suppose it all comes down to me wanting to play full party micromanagement game, and I want it to be as tight and tailor-made experience as possible. I am selfish like that.
I am always in favour of modular difficulty, though I am not sure if the party size is something that should be part of it. Still, I would happily trade some of P:K flexibility for tighter pre-set difficulties.
