For Jerry, Eberflus is a comfort hire. Aristotle talked about how virtues in excess can become vices... Jerry's virtue turned vice is loyalty. He likes his familiar puppet coaches and his familiar players, and he tends to keep both around until it's no longer tenable.
Of course, his real vice is his narcissistic need to be "the football guy" in control, and that's why we've had a 30 year parade of puppets come through in the first place. Let's not forget the big picture here.
Anyway, for Eberflus, Dallas is a lay-low job after his Chicago tenure went down in flames. He just needed to keep a DC headset on more than anything. God knows he won't be a head coach again any time soon, but in a season or two, he can apply for a better DC job as a "current" DC, rather than say a linebackers position coach looking for a promotion.
So Jerry likes Eberflus (as long as he doesn't ask for too much control), and Eberflus likes Jerry (understanding that he was an easy-in to another DC job, and that having little control would be part of the gig). It's a marriage of convenience, and winning football games never really factored into it.