Sakho can already offer what Kolo has and then some. Maybe in the short term he is ahead but long term Agger and Sakho can be a brilliant partnership.
Toure's got years of experience, won league titles (once without losing a game), and knows what it takes to play for a top team at European level, with all those expectations. We need that in our side. Agger has some of that experience of course, Sakho doesn't really - yes, he's been captain of PSG, but they weren't on anyone's radar up until they won the lottery. And he got sidelined a bit after that anyway.
Kolo Toure has won PL titles. I think the importance of that experience is massive for not only our team, but club now. He's a senior player, a leader on the pitch, and an organiser of the defence. Sakho, Agger, Skrtel will learn a shitload from him. As will our senior players. When was the last time we had a player who knew what it took to win a league title? And he's actually still a top defender I think so it's not like the only reason he'd be getting in the side is because of his experience.
For me, Toure's the first name on the sheet when it comes to the defence. He just needs to be in the team in my opinion.