You can't say for certain what anyone's motivations are. Mel might want studio dollars to help fund The Passion 2. Said studio might do so only with the stipulation that he help turn their ailing DCEU around. I haven't seen the slightest evidence that he simply wants to "to attempt something transcendent with a genre he finds mostly mediocre." He hates superhero movies and has said as much. I can only assume that he'd be receiving some other benefit for signing on.
so win win