are you suggesting that picking up a replacement pilot for the onward/return flight from JFK would be a good enough reason for a turnback,
Without commenting specifically on the Virgin incident
At least one operator/authority has a rule that a reduction of the flight crew compliment to below the basic level ( e.g from 2 to 1).
1. Is treated as an emergency.
2. Requires a PAN call.
3. Is usually a "land as soon as is safely possible at a suitable airfield" (taking into account that it might be safer if operating single pilot to divert into a familiar airfield rather than a nearer unfamiliar airfield.)
Commercial considerations don't enter into it.
Hope that helps.