Fidelio has already been staged (and revived) in Munich. Die Soldaten is a brand new production of a long and complex work. So which production will get the lion's share of Bieito's personal attention?
Of course directors often delegate revivals and transfers to other people, and there's nothing wrong with that - unless the show is sold on the director's name, as all ENO productions are this season.
If ENO's strapline "Fidelio - Directed by Calixto Bieito" is currently missing a " 's assistant", now would be the time to add it.