Equally spending more money on Koulibaly or whoever doesn't mean he's necessarily a better pick than anyone else. It's just about lowering the risk, Koulibaly is high risk because it's hit or miss, you ain't selling him on if it goes wrong. Upamecano is high risk too but at least there's a chance of recovering some money if it goes wrong due to his age and reinvesting it on another CB, much like we'll probably end up doing with Stones now.
A younger player simply gives us more opportunities if it goes wrong, edging your bets as such.