It would be nice if they could open up that end of the platform. It would mean ticket barriers being installed. Does that also mean there has to be a member of staff there? There may be issues with pedestrians flowing and in and out of the car park, though. It's not great for pedestrians as it is. And the car park belongs to Lewisham, not Network Rail.
In terms of drop-off / pick up - I always wait in the Perry Vale car park. If the person is fit and able, they walk around to meet me there. If not, they call me when they get off the train and I drive around, stopping at the station door to let them hop in.
People park at the station because the spaces are there. Doesn't mean we have to have them!