Yes $ dollars is the whole reason.
If you look at the CO fleet in two years we will be able to mix and match the equipment on any route, almost right up to the last minute for maxium revenue.
Example, EWR-TPA is a B757-200, and EWR-MCO is a B757-300, but lets say yield Mgt. sees a need for more seats to TPA on Fridays; they just change the airplane routing, nothing else, and there you have we made max RPM's, with very little changing. The same thing will work with the B737 fleet. I may not like having just Boeing's, but hay I think its going to be excellent for the bottom line...
As for the original question, yes the B757-300 will be a good replacment for the DC10 domistic, we are not going to us it internationally on 7Hr flights.