Step 1: The clinical picture is an elderly, postmenopausal lady with the most advanced grade of uterine descent. Because she is past menopause, preserving the uterus offers no benefit and uterine loss of estrogen support drives the prolapse.
Step 2: The aim of treatment in completed-family, severe prolapse is to excise the prolapsing organ and rebuild the weakened pelvic floor. Removing the uterus through the vagina and then tightening the supportive fascia and vaginal walls addresses both the descent and the associated cystocele/rectocele in a single sitting.
Step 3: Conservative tools (a ring pessary) and partial procedures (isolated floor repair or vault fixation) do not give durable cure here and are reserved for poor surgical candidates or specific vault defects. Hence the management of choice is surgical removal of the uterus with floor reconstruction.
\[\boxed{\text{Vaginal hysterectomy with pelvic floor repair}}\]