Soft, mushy stalks are a symptom of root rot caused by overwatering. Have you repotted (the most common cause of root rot) or changed anything else its care recently (e.g. light, watering)? Once a stalk has begun to rot, the only solution is to remove it. If the damage is limited to the top part of the stalk be sure to cut below the damaged area. Also remove the plant from its pot to examine the roots, trim any that are mushy, black or shrivelled, then repot into a size pot that will snugly hold the rootball.

Marginatas are not known for their longevity. If you've successfully cared for your marginata for so many years and have changed nothing that accounts for its sudden decline, it might be that the plant is reaching the end of its natural lifespan. But check for root rot first.