LH and SQ aren't the only airlines in the alliance. And, there is good LH connecting service from PHL on US and UA. It would be in UA's better interest to keep the new US rather than let it go. I don't necessarily agree that just because CO's route structure is stronger that it would be a better candidate for Star. It's not only about providing connections between carriers, but also insuring vast coverage around the globe. For the places CO flies that US doesn't, another Star carrier already does. That's a significant issue. Aside from opening up EWR (which is already well served by UA) and IAH to connections, CO wouldn't really bring much more to the table.

But with the new USAirways, Star would gain more Southwestern US coverage to PHX and LAS with more extensive connection options.