Tech tree - locking Witch Doctor - logic?

In the Medieval Tech Tree, in most cases when an older ability is locked it is because there is a replacement ability that is gained with that particular research.  However Witch Doctors are locked after researching Monotheism, yet the replacement ability Sisters of Mercy is not available until researching Charity which is further down on the tree (and is also a forked research pathway).  This seems illogical and inconsistent. 

Bers why don't witch doctors just transition into nuns once the research has been completed?  This would also allow you to keep your nuns instead of switching back and forth between them and pilgrams.   After saying this I know that we don't know what's coming with the next stage and there might be a reason for doing it the way its done.