The more you have on your party, the less each of them counts. This is common sense, not an opinion.
And I personally would not like more less important members in my squad. This is an opinion.
Also, I personally would absolutely not like to manage six folks. Personal taste.
If you want to maintain a certain level of high quality overall, there is no balancing for 'well, take 3 or 4 or 5 or 6'.
That is quite obvious and needs no arguments I believe, we are all thinking people.
Thus, for the best this game can be it should be optimized for what the makers envisioned it to be - in my understanding.
In an optional mode - besides the intended version of the game - there could be either an 'easier' mode where you can just throw more in more companions, or enable some auto balancing on your own risk,
without having someone to be responsible for that it works out equally good as the intended way all the time.
Have to say i'ts almost amazing how much I dissagree with pretty much anything you said.
Both in terms of personal opinions and talking about "logical conclusions" that aren't necessarily true at all but you try to state as matter of fact.
Like the suggestion that you couldn't scale things up with exp or something because every fight would need to be "fine tuned"... As if any fine tuning was even possible at all, to begin with, in games with this amount of variables.
Checking any forum discussion about DOS 2 would be a perfect example of that: with people commenting on the same encounter and describing it alternately as "Too hard" or "too easy" based on their setup etc.
