lower runner panel is flat originally, deform under weight of door lack of lube, grinding paste action and rust from inside the sill, eventually bearing goes through the floor
I cut a piece of 1.2 stainless steel to fit the flat area in the runner aperture. It took a bit of fiddling but it was worth it. I bonded it into place with ms polymer adhesive. Looks tidy and has made the door action smoother and quieter. Lasted ok for the last 4 years and no sign of wearing out.