Are you joking mate?
Koulibaly belongs to the very best central defenders in the world. Stones is not even the best CB in Manchester.
You hopefully saw when Laporte was brought in a couple of years ago that our defence improved remarkably. Hell even 19 yo Garcia was an upgrade to Stones, why else would Pep play him? I can take it even further: our backline looks even more solid with a 35 yo CDM (Fernandinho). That’s how far down Stones has end up in the pecking order.
Koulibaly will now partner Laporte who is clearly our best defender. If Koulibaly and Laporte stays fit, we’d have the best centreback partnership this club has seen in many many years.