I was discussing this same idea with StratComm yesterday. I think it is very much possible that the Shivans, either on their own or as tools of some far-off masters, are punishing humans and Vasudans for their innate violence, not subspace use. They came twice during periods of pre-existing strife, and left when the strife ended. The collapse of Shivan forces at the end of FS1 wasn't a retreat, it was a reprieve, and the supernova in Capella may have been a reminder of the consequences of being "destroyers" ourselves. This also fits the "symptom of a bigger problem" (the proclivity of humans and Vasudans to violence and xenophobia). If a war occurs between Sol and the GTVA after the Terrans build their jump gate, we can expect the Shivans to attack a third time.