1) A new subfleet? Obvious benefits -- opens up the new markets. But also obvious costs -- complexity, scheduling, etc. Recent trend has been for fewer subfleets, not more.
2) More 757 routes. Good, I guess. But this means the pressure is on for existing routes to defend themselves. I guess AA picked up a 757 from cancelling BOS-SJC. (Not to mention other cancelled transcons.) But every plane has to come from somewhere (putting aside further scheduling improvements), which means every route addition entails a reduction elsewhere in the system.