A method and system for controlling air-interface connectivity of a user equipment device (UE). A computing system detects, when the UE has dual connectivity including a first air-interface connection with a first access node and a second air-interface connection with a second access node, and when the first air-interface connection is defined on a first carrier, that the UE is moving toward a geographic area where UEs that were served by the first access node on the first carrier as part of dual connectivity lost their dual connectivity due to their having insufficient aggregate dual-connectivity throughput. And based at least on the detecting, the computing system reconfigures the first air-interface connection of the UE's dual connectivity to be defined on a lower-frequency second carrier, to help avoid having the UE lose its dual connectivity.