Well, siva brought sepiks back, why not taniks? And the vex can travel through time. They don't see time in a straight line as us, even with the restorative mind destroyed, who's to say they don't have more or didn't grab a new nexus from an alternate timeline. Even the bald chick said something about the vex messing with time and if they caused the collapse and shit. With the above true, they would also be vulnerable to being taken. If the nexus knows it was destroyed, it could be taken by being offered and upgrade, which you see in the taken version of the strike. I get was your saying about strikes, but given vex time travel, and undead and reality warping capabilities of the hive. They would be most suitable for repeated runs of strikes. Or raids even.