Nah, I see where you're coming from with that but I guess it's because they really wanted to get that "hope" message across in the movie. I felt like that was a theme they were really trying to highlight all throughout the movie and the death that occurs helps in doing so.IMO if you're going to go and try to go completely like the comics, but do it in modern times sort of thing, why would you just go overboard and kill off a key character? And why add Rhino to the poster if he's only in it for a mere 2 minutes, not even a full fight.
But yeah, I was pretty disappointed about the set up for the sinister 6 in the end but I guess it was a prelude of what's to come next. I remember watching old cartoons with Spider-Man up against the sinister 6. I saw Rhino and got hope but it was a let down right there.