They didn't have any trouble filling the 717 last summer on MKE-DCA, yet things were still apparently so poor that they chose to give up the free DCA slots the DoT had awarded them rather than continue to fly MKE-DCA over summer.
It certainly will be interesting to see how all this new service does. What most casual observers don't seem to understand is that the Midwest pulldown in Milwaukee was targeted, not broad. Midwest's reductions handed a ton of traffic to AirTran in leisure markets, and AirTran has filled planes well in those markets by backfilling with far less seats so overall capacity is still smaller than it had been the prior year. But Midwest has more seats to Boston than last summer, more seats to LaGuardia than last summer, nearly as many seats to DCA as last summer, and (between YX and NW) more seats to Minneapolis than last summer. There is not a corresponding YX pullback in any of these markets handing them traffic as they received to MCO, TPA, RSW, LAS, etc.