So while the midfield helps in shielding a defence, a solid defence also lets the midfield be more aggressive and gamble more. One should not be neglected one for the other. A top team should not be found lacking in area of the pitch, otherwise soone or later that weakness will be exploited by others 9see liverpool recent struggles due to their limited midfield).
The quality of a defence is not always evident from stats e.g. LvG and Pep defenses look better than they really are cos they play a possession game that limits how often the defence is tested. Both are actually poor defensive coaches, and guardiola in particular simply throws money at it, having bought failures like Chygrynskiy, maxwell, caceres, and henrique.
If you have quality and experienced defenders, then yes it easier for them to figure it out on their own without anyone needing to be a clear leader in the sense, but when you have younger and less proven players, the presence of the leader makes a big difference.
At Madrid Cannavaro was playing alongside Marcelo, Ramos, and Pepe (all<25 back then), who were much younger and less experienced and thus his leadership and mentoring was crucial. We are currently buying younger and less proven defenders like Bailly, Lindelof, Dalot and Shaw, thus the need for an experienced leader. (Note, I think it is the right approach as it is cheaper and will likely produce a longer lasting group of defenders than spending more on finished products when the funds can be used in attack and midfield where talent is more relevant than experience)
If we are going to be buying young unproven defenders, we need to put in place an environment that would ensure they mature in the right direction and at the right pace, and not leave them to simply figure it out. Right now, that Smalling is our most experienced and possibly best defender does not bode well for the development of the younger ones.