Carsley said he didn't want the job because he didn't feel ready for it. Since then he has won another U21 Championship and had temporary experience in the job itself. By the time Tuchel leaves I imagine Carsley will be ready to take the job on. Southgate stepped up from the U21's so there is previous in making the step up, and given Carsley has been far more successful than Southgate was in that role, and that many of the U21's he has successfully managed are likely to be features in the England team, he seems an obvious choice to me.