此消彼长

(idiom) one declines while the other flourishes; one loses out when the other one gains (as in a trade-off or a zero-sum game)

length / long / forever / always / constantly

chief / head / elder / to grow / to develop / to increase / to enhance

this / these

this; these

to disappear / to vanish / to eliminate / to spend (time) / have to / need

that / those / (one) another