I could add some words to Nasus's kit if that addresses your concern. You might have noticed that I can be long-winded.
Really though we consider a bunch of factors when releasing champions. Themes, playstyle, popularity, diversity of the class, diversity of their play position.
You might say Gwen is unnecessary, but I say she is an AP Baronlaner, with a killer theme.
You might say Yone is unnecessary, but I say he is a very popular champion with an appealing theme and flashy playstyle.
Warwick and Vex are both great (I worked on them both, I'm glad you are excited for them), and they touch on many important traits too. But not every champion is going to appeal to you, I would much rather ship champions that some people love and decide to main, and others hate and ban, than champions everyone feels ok about.