Harris is weak making trades. It's like he's afraid any prospect he may trade will turn into a really good player. And any player worth trading for usually comes with a big salary.
And then there's his free agent signings. Yes, Torres has been good. But was it a case of his talents being offered to the Tigers or Harris actually going after him? Seeing that 2nd base hadn't a been a need and the signing was a surprise, I would say Torres fell into his lap. Cobb at $15 million? He could have had Matthew Boyd for the same money per season.
Harris has been really good rebuilding an empty minor league system. But there's more to being a successful GM than just that. He had an opportunity to have a deep playoff run, this season and it looks like he's failed. He'll be kept around because the Tigers are making the owner a lot of money. And that's what determines if a business is successful or not.