they do not have to sacirfice an entire faction as the bad guys. they already have tyranids. there are also other minor faction that can be used form time to time as diversity
and the simple truth is EC is a PvP game, the fact it has any PvE at all should be seen as a huge allowance and their choice of Tyranids as an NPC faction allows for quite a lot of depth is possible scenarios, add that at present the NPC faction will only appear in underground "instances" rather then in the open world means its a side-show to the main event, Personally I'd much rather see more player factions then having more NPC factions that would be stuck in instances like the 'nids
