In HCV, where the "party" is going to last only a short time (at least in the western world) and you can only get the patient once and possibly for as short of a period as several weeks, I think there is plenty of incentive to form as many collaborations as possible as early as possible.
I think your logic here is contradictory. If they were to partner with BMY's NS5A they would be forced to split profits from a limited amount of HCV patients in the developed world.