Which is exactly what we had been doing for decades. Some people seem to forget that when we bought Stam, Rio and Vidic they weren't exactly the best defenders in the world. Stam was a highly rated CB who had never played outside of the Dutch league. Rio was signed 2 years after SAF ridiculed his fee at Leeds while Vidic was heading towards Fiorentina (not Real, not Barcelona, not Juventus but Fiorentina) up until we snapped him up at the 11th hour. You can't expect that a new signing would carry his entire department by himself. What you can do is improve that department with the talent you need while hoping that 1-2 of them would step up and make a huge difference.
I think that gambling everything on Koulibaly would be a big mistake.
A- while he's an excellent defender he's not a one man army. Don't expect him to cover everyone's arse like a Baresi would do. He's not that great
B- our defence need alot of TLC. The RB position is in shambles, we only have 1 natural RB whose a kid. Meanwhile the LB role isn't faring that great either. If Shaw goes back to the treatment room then we'll be in big trouble.
So instead of blowing our entire budget on 1 CB then lets properly strengthen the backline first. If we do that then we would be less reliant on the individuals and more on the collective. Which is exactly how proper top defences should operate.