Hitler left the Church, no need to excommunicated him as he was already gone.
Saying any number of prayers does not help the person who is in a state of mortal sin. Mortal sin separates a person from God, severing the relationship between them. Excommunication is just an outward statement of the already-existing reality.
Notice that Archbishop Cordileone said she has to *publicly* repudiate her current stance before she can be re-admitted to the sacraments in his diocese.