CW shows have killed enough lead characters, The Vampire Diaries and Supernatural alone have made sure of that. If a character is popular, the actors wants to continue and it doesn't make sense story wise, than why kill of a character? Because that is the case of Felicity. Sara is what you get when it makes sense story wise, but the character is popular and the actor wants to continue.
First of all, the story always makes sense, if they want it to.
That said, you're right, of course. If everyone is happy with the character, then there's no need to kill it off. But saying they can't kill off a character, because she or he is too popular always feels limiting. If the writers have a great idea around Thea or Diggle, that really impacts all characters and the story going forward, but can't do it, because fans would get upset? That would suck.
But it's not like we'd ever know about that. It just sucks if they keep characters around, shove them into every episode and don't give them a purpose or an arc.