my wild guess would be they do not want the disparity on number of party characters between single player and multiplayer. if single player proven much more enjoyable than said multiplayer with 6 party characters, they would be slapping themselves in the face. restricting it to 4 makes the game more accessible to general audience. having more party characters would be a nightmare for them especially if larian's goal is to market the game to multi-platform be it on consoles and handheld. it certainly won't be an enjoyable experience for casuals especially if they have to control too many party characters.
that is the sole reason why they want it to gimped the game and forced it as a party of 4 (which you have exposed them of gimping the stats). best hope really would be mods but i really hope larian will really do the game justice and allow 6 party characters. looks how successful with pathfinder wrath of the righteous. they should learn from owlcat honestly.