It's hardly going to wreck his reputation or bury his future chances of getting a job if he does take over and they go down. He'll still be Champions League winner Rafa Benitez, not Rafa "the Newcastle relegator" Benitez. I mean he's still highly regarded despite failing at Inter, Napoli and Real Madrid.
He's always retained his home in England ever since he left Liverpool, he likes living and working here, he isn't going to be in the frame for anyone in the top half of the Premier League at the moment, Newcastle is one of those challenges where if you get it right (as in Keegan's first spell or Sir Bob's time) then it's great. Ashley actually seems to be funding them properly now as well, his mistake this year was employing a born loser.