I think the only valid reason for not implementing Multiplayer, as it was said, is development effort / resources that could be put into something else. If MP was implemented, obviously no one would be forced to play it. But it can be argued that adding full Multiplayer, Custom Battles+Campaign, will mean that this game will target bigger audience - more people buying the game, meaning that Game Labs will have more resources to invest into this game, possibly putting the development forward, rather than if MP wasn't implemented all.
Now balance isn't much of an issue, people still play paradox games like Hoi4, EU4, CK2 and so on... even tho you start as much weaker nation. Not because of balance, but because it targets certain themes like ww2. Now i have no idea if Modding will be ever added in any form in this game, but if yes, absolute balance can be achieved.