They're built by selling players and buying worse ones to ensure some arbitrary idea of balance?
You've said if Hummels were available we should sell Jones and then buy someone not as good as Jones to be a backup player and ensure we have "balance".
When has any side ever done that?
Everyone did that. We did it with Pique for example. He was surplus to requirements so we sold him. Because we had Rio-Vidic as a main paring. The simple truth you are so blindly cant see is that every top team has a solid two CBs, one just as good or a bit worse for rotation. And one for deep cover. Or two main CBs and two cover players.
Real is built that way: Pepe-Ramos - main, Varane - 3-d choice. Nacho is a deep cover.
Munich: Dante-Boateng
. Martinez for rotation and Buyten as a cover.
Roma: Castan-Benatia, Burdisso and Toloi are cover, played less than 10 games between them.
Dortmund: Sokratis-Hummels-Subotic are main three due to injuries and Friedrich is a cover.
Chelsea: Terry-Cahil, Luiz for rotation. Ivanovic played only four games in a CB role.
Atletico: Godin-Miranda, Alderweireld is a cover.
Barcelona: Pique-Mascherano, Puyol was not fit, so Bartra was a cover.
That's how a team is built.
Three skilled CBs is more than enough, some teams even do well with
two. When you have four CBs of the same level and you keep rotating them, it never works in your favor. And defense needs stability.