Post menopausal. have 9mm thickened uterine lining that biopsy said is benign. no bleeding. is hysterectomy a radical treatment for this condition?

Yes: An incidental finding of a slightly thickened uterine lining is not a major emergency. It sounds like your doctor has carefully evaluated you and not found a problem. Unless there is more to the story that I don't know, a hysterectomy seems like a bit of an overkill--like cutting a tomato with a chain-saw when a paring knife would do. Best wishes!