Well, here is my take on this subject,
At the beginning , because one of the main complain on Pillars Of Eternity was : "Combat Readability".
They chose to reduce that "party members cap" from 6 to 5, to solve that problem.
But, after a while, and I think "very recently", Devs came up with that "VFX Opacity" Feature... which, pretty much solves Combat Readability issue on its own, I think they kinda "didn't see that coming". Moreover, now that the Game is fully scheduled down : (Scale wise, Character wise, Monster wise, XP/Level wise, etc...)
I don't wanna say it's to late to change things back but you know what I mean. It probably is...
I'd rather want 6 aswell, I enjoyed Tyranny's 4 a lot, & I'm ok with 5 !
Let's just see how it feels when the Backer Bêta comes out.