Metrojet draws is fleet from the mainline. While MJ expansion was warranted, and attempted last summer, expanding too much meant that there weren't enough planes for mainline to fly, so the scale of MJ was brought back. You would note however, that SW has been slowed very dramatically in US Airways territory by MJ as MJ competes out of every airport except Islip and very recently, Buffalo. Islip service however was never a big market for US as they focus on LGA and to a lesser extent, EWR. All nonstops out of BWI (SW's big northeast hub-like city) are matched by MJ, and SW does very little point to point flying in the northeast. Almost all flights are to or from PVD and BWI.
Feel free to correct me if I'm wrong.