the plant itself is not dying - the defoliation you're describing happens for various reasons, but most usually temperature fluctuation - Ficus hates changes in temperature and draughts, and will usually drop some leaves in autumn and spring as the seasons change over anyway (even though they're indoors). Drought or overwatering can have a similar effect. You might try cutting back the bare stems (make sure they don't drip the milky sap on furniture or carpets) in March/April to see if that encourages new growth, together with repotting into fresh compost at the same time. Also, examine the plant for signs of infestation or disease. Otherwise, when they get to this stage, most people replace them.