True. But it was just another part of Eren's foolishness that he assumed all titans are trudging, dull eating machines. It's all up to luck, I guess, and it's fickle in the battlefield. For anybody wanting to survive for longer, it would be best not to count on it. Nothing says Eren wouldn't have encountered easier titans later, and similarly Mikasa might meet the tougher ones, yet all the same they would need to be ready to face and elimate them all.