You say Fernandinho offers security, leadership and experience; I'd say that's the perfect blend to help the likes of Stones, Garcia and Adaradiayo prosper and develop into the long term partner for Laporte.
Koulibaly can only be world class when he's on the pitch too, unfortunately that's not very often recently. I certainly don't think spending £80m plus (it's Napoli), is a wise decision. It's a humongous risk which could potentially cripple us with FFP restrictions when we've got Aguero more than likely leaving at the end of next season, combined with what will probably end up being a low fee/free transfer of Sane who we may very well need to replace, plus Fernandinho, plus a lack of depth at LCB and left back.