While I certainly do miss the potential for a big sprawling party that the old BGs had, I do see some issues in increasing the party size further. In no specific order.
1) If there are 5 touring companions then who exactly are we not bringing along? With a party size that big then we either always have a full party with the same faces, which is not a whole lot of fun, or we need a *a lot* more companions. And more companions equals a lot more work.
2) With two more party members, group control becomes horrendous. Imagine six different characters that all have a summoned critter of some form. 12 different creatures just squirming around? That would very badly need to be sorted out somehow, which is non-trivial. Currently, with 4 characters it might just be possible to live with it.
3) Four characters and group / ungroup mode works reasonably well. The hotkey is unbound by default, but I am finding it extremely useful to just ungroup whenever I need specific control over positioning and regroup when I just need everybody to come along. But with six characters, it would end up being a lot of clicking. Basically, there's sort of a way to bypass the control issues but that way gets a bit rougher with more characters.
4) The gameplay is already a fair bit complex. There's a lot of stuff to constantly keep in mind, a lot of positioning and potential positioning to consider, and a lot of nursing the fragile characters. Having two more characters in the party (and their summons, potentially) makes that worse. I could overcome, you could overcome, probably most people on this forum could, but Larian has to design for a wider scope than that.
5) Given the turn-based nature of combat, a bigger party makes combat a lot slower, as more characters have to be part of each full cycle of turns. And combat can already be a bit on the frustrating side of slow.
6) Rebalancing. Lots of rebalancing. A lot of playing through with different party sizes will be needed to figure out if the balance is where Larian wants it to be, regardless of group size. Enemy group size, amount of enemy abilities, item economy, character targeting, there's quite a lot that goes into making an encounter "feel right". I do not think this rebalancing can be done through automation, either, so that's yet more work for Larian.
7) The game and the gameplay so far works reasonably well with a 4 man party. If it works, it works. Spending too much time on something that works risks not having the time to fix something that really doesn't work all that well.
And I would like to stress that probably neither me nor Larian themselves mind them having to do "more work", but time and resources are finite and there is an object lesson to be learned in Cyberpunk 77. I would personally prefer that they get the fundamentals of the game right and actually get around to building a wholesome game, even if I am limited to a 4 man party, as opposed to getting distracted and then having to rush the last half of the game.
Also, since people around here seem a bit testy, I will add that I am aware that points 2 and 3 basically boil down to a different problem, but that problem is so far unfixed and we cannot be sure it will be resolved properly. Without a resolution, I feel it causes further problems for an increased party size.