Yeah so story missions and story mode for certain flashpoints.. that's not 95% of the game. You're ignoring veteran and master mode flashpoints, operations, warzones, 4 man heroics, and Ilum (Haha jk). TOR wasn't designed as a single player game with an optional multiplayer mode. It's an MMO.
Oh there's also the Eternal Championship you can do that's kinda like the Brawlers Guild in WoW. That's fun.