Eren is the best protagonist for a series like Attack on Titan.
A lot of people get frustrated that Eren is slow to jump into fights, like when he couldn’t transform to fight the female titan and when he froze up last episode during one of the Top 10 Anime Betrayals of all time. I think this misses out on a crucial point. Eren is actually very quick to join the action - but only when the situation fits the narrative of a sterotypical shounen. He’s the first one to go after killing the colossal titan when it appeared in episode 5. He saves Armin from being eaten alive while bleeding and missing a leg. Getting revenge for his mother and saving his childhood friend - classic shounen plotlines.
Eren is, essentially, a sterotypical shounen protagonist trapped in a storyline that subverts shounen tropes. He’s in the wrong genre. He views the world as very clear-cut good vs. evil. Humans are good, titans are bad, and humans will eventually succeed because they are on the good side. He believes it, so we believe it too. This is what makes it so shocking when the story deviates from shounen norms. (see: Levi’s squad and how the power of friendship failed to save them)
This is also the real reason why Eren hesitates when it is revealed that his friends were his enemies all along. It’s not because he’s a wimp or because he’s the new Shinji, it’s because his character isn’t prepared for a morally ambiguous world. However, this is what makes him a such a great protagonist for us. While we come to terms with the fact that the world of Attack on Titan isn’t as black and white as it first appeared, so does Eren, right alongside us.