First, anything more than a party of 4 makes an already tedious process, well, er, more tedious.
Second, currently, EA Act 1 can easily be wrecked with the current party size....having the party size increased only makes the game what, more easy to be wrecked? If I, as with other solo 'lonewolf' players, can wreck EA Act 1 with one lone solo character/toon, why on earth would one want a party increase beyond that which is already established?
Well there is a few reasons ...
Every single of them allready mentioned in every single topic about party size ... there is quite a lot of them also, wich can be concidered another reason.
But most of all, why not?
You confess that you managed to win whole game with single character, that means if you are using whole party you only get the same faster ... since only advantage you get right now is combat (since other characters never partipiate in conversations) ... so players will be able to finish whole game even a bit faster, why is that bad thing?
If people want something and it makes your game stupid and breaks logic or laws ... its compeltely fine to say "no".
But what exactly does this change ruin?
What is reason Larian is fighting so hard against it?