megan, the senior cabin crew can deal with any matter that they have listed as their responsibility (legally) as per say SOP's.
If a matter is not covered by SOP's - lets say "de boarding" a passenger involuntarily.
The senior cabin crew is not legally allowed to de board as per a SOP and can then be personally responsible for what ever may happen.
So to have a person who can be legally responsible and have authority at all times is important.
In this case it seems we did not have an overlap in legal responsibility but had a gap.
The gap is easy to fill or may already be filled - A simple entry in the SOP's
" At any time on the ground or during flight the elected pilot in command can assume responsibility that is not covered by a SOP or regulation".