I think it's a valid point that Attack on Titan's main characters are quite generic and overdone, as shown by the blonde, geeky or nerdy sidekick COUGHARMIN, and The Thirsty Lowkey Yandere coughmisaki who is overpowered female. Then there's the issue with Eren himself. I would admit that at the start his personality was just a rehash of every other shounen anime protagonist who has a revenge plot in mind, and it was quite bothersome to have to watch him disregard everyone else and the mission for his own selfish desires that turned out to be quite irrational and childish at times.
But character development happened and Eren actually now seems like a half decent character now. At least, in the manga. The point about Attack on Titan is it's not so much a 'character driven' narrative, as you mentioned, because the characters just happen to be there to represent plot points for the narrative (manga spoilers but yeah). The interesting thing about Attack on Titan is the philosophy and world building behind it, which I think is quite well done in terms of depth. I'm assuming you're an anime only watcher and that's just such a shame because the manga goes into so much more detail in terms of world-building and character development. Overall, the Attack on Titan character cast isn't really what Attack on Titan is known for, it's the world building and the character development of the main character as he matures into the role he is forced into.
As for the matter of killing characters off, I think it's just one aspect of the story that has to occur, since Attack on Titan is a series about humanity's survival, and obviously there's going to be sacrifices in the thousands. It's not a story set about for a particular number of the cast, it's a story directed for 'the rest of humanity' and we just happen to be centering on Eren Jaeger who is central to the advancement of humanity. So in retrospect, having lots of characters die also give a sense of emptiness which is reminiscent of real war in the world.
Basically I'd argue that it's not the characters that draw people in, it's the narrative.