I am kinda on the OP's side in this - but with 1 difference, A3 would NOT work as a MMO . . . . .
As any1 who has played Planetside will know, the fun taking bases and towers soon all fades and gets repetitive, and here-in lies the answer on why A3 would not work.
For a successful MMO to work 1st and foremost you need players, players to keep a subscription - to do that the gameworld has to tie itself to the player, involving them and giving them something personal to work towards - in order to do this the game would need to enlist certain MMORPG elements such as choosing a class and leveling it up.
Say, for instance, you choose infantryman, you would start with the basic generic weapons and, through completing missions, would gain xp and points to branch off into being a sniper, medic, demo etc etc . . .
Then there are the missions, quests, if you will, that you would need to complete in order to level up - missions that would proberly be designed with a overall story in mind and mission chains etc etc . . .
I have many an idea on the subject of a MMO, based on a game such as ArmA - some qualities of the ArmAVerse certainly lend themselves well to a MMO-MilSim, but thats exactly it, some qualities . . . .
ArmA3 as a MMO = boring repetitive pvp/AI
A MMO based on ArmA3 = ever changing pve/pvp/AI scenarios