I don't think it's inevitable that Cav joining Sky would kill their GC ambitions. Sure, it's unusual for a team to be successful on both fronts, but far from unheard of (see Zabel & Riis/Ullrich).
It would slightly limit their options in the mountains as they'd need to sacrifice one team member to escort Cav through ahead of the broom wagon, but with super-domestiques of the quality of Thomas and EBH, they've got riders who are capable of both leading out a sprint and riding to support a GC contender in the mountains. And if Cav joins Sky, surely he'll want to bring Renshaw and Eisel with him?
Imagine... Wiggo, Cav, Thomas, EBH, Flecha, Gerrans, Uran, Renshaw, Eisel.
Sky's problem long-term could well be keeping the fringe members of their already large roster happy. They've got the likes of Pauwels and Lofkvist on the sidelines - two of their strongest riders in last year's Tour who didn't make the cut this year. Henderson has grumbled on twitter about not getting picked. Cav joining Sky would probably put a dent in Ben Swift's ambitions. And when will Peter Kennaugh get his chance?
d.