There's no scientific evidence that Benatia is so much better than Luiz. All I know is that 1 year ago most football fans didn't know him while Luiz was a member of the best defense in the Confederations Cup and that's with the likes of Alves or Marcelo starting who are more winger than defender.
Most football fans are not the best informed, or are in tune with outside the renowned. Quality is judged by way of consistency, not over an instance.
Benatia beats Luiz in every stat that matters and that includes PS. I'd post the stats myself but since you've called arguably the world's best CB as overrated, in order to make a case for David Luiz. I will just save the time. And its not so much about making errors (no one is infallible, neither is Mehdi), but about minimizing and regulating them.
I have to admit, his long shots will help us against buses. His free kicks too, if he ever gets to take them. Not to mention his heading ability when we get corners.
For some one who's regularly hailed for his heading ability, his aerial stats aren't particularly flattering.
As for his fks, one Chelsea fan wrote:
Not to mention we won't have every free kick blasted into row ZZ anymore.
Another:
Sell him. Too much money to resist for a player whose next brainfart is just around the corner.
The general sentiment of their fans seems to be that they'll miss his personality but not the footballer.