Landing retros might save some mass and volume over an airbag system, but the extra flotation 'stuff' needed for an emergency water landing (as opposed to an 'all-in-one' airbag landing design) might just eat up any weight and volume savings.
How about not ejecting the heat shield but design crumble zone into it to cushion landing? Or the good old Mercury style of dropping the heat shield to hang from one meter long flexible tube and let it form the bottom of landing airbag?
I'm glad they are sticking with the airbags. There is a lot less to go wrong.