You make this move now, in January. When we are one of the favorites to win the CL, and are relying on 32 and 34 year old CB's for the vast majority of the remainder of the season.
So, let's use your argument here, shall we? Let's say, for shits and giggles, we sell Benatia for 7 million in the summer. That's a difference of only 3 million Euros, and we are guaranteed a rock solid backup to our starting CB pairing.
Now, we get 10 million Euro, which we may not even spend, lose a valuable piece of the CB rotation, and now will have to rely on a 24 year old who can't string it together for any prolonged stretch of time, and a CB, who A) can't even get on the pitch for Lazio, and B) when he plays CB, is actually a pretty fucking lousy CB.
Makes sense to me.