I think that many people would agree that the Shivans are at the very least the most powerful force that exists within the galaxy the GTVA inhabits. Their technology enables a complete mastery of subspace, and the ability to project overwhelming force to any point of conflict. Now, that being said, we may know a great deal about Shivan technology, but very little of their actual motives. If the Shivans simply wiped out races for using subspace in the first place, the GTA and PVN would have been exterminated soon after they discovered the technology. The empire of the ancients existed for at the very least a few centuries before being wiped out due to some perceived great sin on their part.
Now, we can surmise then that the Shivans do not eliminate races simply for using subspace as a means of travel. And for those arguing that its the sheer volume of subspace activity that is involved in war time, I might point out that the Terran Vasudan war is known to have dragged on for many years with neither side gaining much in the way of territory. Its more likely battles dragged on in one system for weeks or perhaps months. While reinforcements were surely sent in by both sides, I imagine these forces were slow to arrive, accounting for the stalemate.
Increased subspace travel would also be much more common in peacetime, or at the very least far above normal. Why you might ask? Simple, colonization. With no wars to fight, industry turns toward expansion. Explorers set out to chart new worlds and nodes, and suitable systems are often seen as the land of milk and honey by colonists who may be leaving behind rather dismal conditions in the more largely populated systems. With no enemy to worry about, explorer vessels can load up on supplies and head out as far as they can travel before returning home. In their wake come colonist, entrepreneurs, and corporations, all looking to gain something out of these new discoveries. The flood of ships would be much more numerous in such a case.
Case in point: the end of the Great War meant a time of rebuilding for both species. Industry and manpower were probably moved around a lot during the Reconstruction, meaning travel probably increased from wartime. A good example would be Capella, which somehow managed to build up at least 250 million colonists in 32 years or less depending on when it was colonized. That's a huge population boom for one system, so immigrants had to be arriving on a regular basis with the population of the other colonies was still increasing.
I believe we can conclude then that it is not an increase in subspace activity that draws the Shivans. If it were, the GTVA would never have been formed because both species would have been wiped out in their infancy.
Any species that discovers subspace technology and moves out into the galaxy is not then automatically marked for the destruction. Therefore it is quite possible there are other space faring races in our galaxy, though where they are is anyone’s guess.
It would be interesting to see a campaign dealing with two new races facing the same problem the GTA and PVN were during the T/V war.