You can still get duffers in such jobs as often a candidate is put forward who's agreeable all parties and as such can be the lowest common denominator who won't rock the boat.
In practise, most german presidents will just be way too old four years after they leave office to be a viable candidate, and it looks a bit absurd for them to be pushed back into the office, exactly because there's very little real power.
I agree though, it's not an issue, precisely because the German president has historically not acted partisan (even though he's usually a member of a party) and not be involved in day-to-day politics, so it's not too hard to find somebody that everybody can agree on. This may change if the office becomes more politicized, but I doubt we'll see somebody get re-elected after having paused for a term.