Complex sentence: Concourse D, which would have been an island concourse connecting to Terminal 3, was in the planning stages before Delta's bankruptcy, in order to deal with an increasing number of flights, however, due to a decrease in air travel, was never built.
Split into simpler sentences: Concourse D, which would have been an island concourse connecting to Terminal 3, was in the planning stages before Delta's bankruptcy. It was intended to deal with an increasing number of flights, however, because of a decrease in air travel, was never built.