Herra: I agree with your point there, although I don't think that it's apparent to the GTVA as of yet (should it be? Perhaps the Ancient monologues were never fully studied?). In their minds, I'd guess that it's a question of "When the Shivans return," rather then "Why do they keep f*cking coming back and how do we motherf*cking stop them?"
It's hard to say what the Ancient Monologues actually were
. They could have been recorded pieces of Ancient history from the sources recovered during the Great War, or they could have been exclusively experienced by the player character - Alpha 1 of the Great War.
If the former is right, it's hard to understand why they would not have been extensively studied. On the other hand, if they were exclusively experienced by Alpha 1, GTVA would never have known them since Alpha 1 was stuck in Sol. This is, in my opinion reinforced by Alpha 1's campaign ending monologue which is spoken entirely in the first person.
Lacking this information, it is easy to understand why GTVA would rather prepare for the next Shivan onslaught than try to remove the Shivans' reason to come after humanity again. It's not a logically wrong conclusion, but one borne with insufficient information.
The Ancients monologue speaks of a specific crime, a specific sin. "Now we know our crime was sin."
It speaks of such sins after their imperialist expansionist tendancies, so it is at best an extremely tortured construction that they were referring to such things. Instead, the closest that would come to an identification of sin is the trepassing.
Subspace is also mentioned in more than one of the monologues, three of them in fact, but imperialism only one.
It is, of course, a possibility that simply usage of subspace will attract the attention of Shivans. It is possible that subspace travel has prolems that are unknown to humans and Vasudans, but Shivans are aware of and try to prevent them the only way they can, or maybe the simplest.
In this interpretation, subspace nodes would be weak points in space-time structure and subspace travel through them would exacerbate the problem when the vortices would poke holes in it even further weakening it. This would further explain why Shivans themselves seem to not be dependant on nodes - possibly they can travel without them.
However, they do use nodes when it is convenient to them.
Or it could be a method to get someone to explode something big (like a Lucifer or a Meson bomb) inside a node in order to collapse it.
This however presents a new problem - if it is subspace travel that attracts Shivans' attention - why don't they simply go and collapse the nodes themselves?
Why the interest in glassing planets (prime example being Vasuda Prime) and the overall destructive nature (exploding a star in a system full of fleeing refugees)?
Of course, like I said earlier it's just possible that Shivans do it for the lulz. In that case, that would be their sole motive, making them truly impossible to negotiate with. However, I sincerely doubt it and think they have some underlying reason for their selection of targets, and it isn't just subspace travel.
Alpha 1 lays it out. Humanity survived, the Vasudans survived, because they were strong, because they were prepared and capable of fighting for it.
This invokes the question: Could they have survived, had the not been capable of working together to defeat their common enemy?
And both these ignore the vital point: It doesn't matter whether you're imperialist or not. It matters only that you be curious. The Shivans do not arrive from extradimensional unreachable homes to destroy those who break their rules. They did not for the Ancients and they did not for FS2. They are here, now, and they await the unwary as they awaited Bosch's effort to find them. Even harmless exploration could bring down the wrath of the Shivans upon you. Unless you're posisting that Ubuntu is so insular it will never even explore beyond known space, some day they will meet the Shivans once more. And they will not be ready for it, they will not be able to buy they few days necessary to throw a pile of Meson bombs down the node and render themselves safe again.
I don't see why curiosity alone would cause Shivans to annihilate your species. Unless they indeed pretty much just do it because they can.
Besides, how do you know Shivans don't
arrive from extradimensional unreachable homes to destroy those who break the rules? In Blue Planet continuity there is already two confirmed cases of inter-dimensional travel - the 14th Battlegroup to the universe where Lucifer glassed Earth, and Sanctuary coming back to FreeSpace Proper universe (BP continuity, of course). So, given that in Blue Planet continuity interdimensional travel is possible, that sort of makes your argument less solid.
As far as being ready for Shivans goes - UEF and GTVA are pretty much on par on their level of ability to seriously deter Shivans should they desire to annihilate them. GTVA might survive for some time longer (mostly because they are spread wider and could slow Shivans with collapsing nodes) - but then, safety from Shivans by collapsing nodes is questionable at best, knowing Shivans' cited ability to rebuild nodes, and even before that, their appearance in Ross 128, likely through a node undetected to GTA observations.
If Shivans decided to destroy humanity and Vasudans, there would be no stopping them. They would find ways around stopgap measures like destroyed nodes and such.
If they do it for the lulz (or, in your words, just to punish for simple curiosity) all is lost, and I don't believe in the no-win scenario.
Therefore I prefer to think they have a reason for doing what they do, and that's either the Subspace Argument (subspace travel is dangerous and Shivans prevent it, thus saving the universe) or the Rulebreaker Argument (Shivans monitor activities of sentient space-faring species and nullify those that present significant enough threats to other space-faring species).
Maybe not the strongest argument for it, but hey, this is a fictional universe with no confirmed canon solution to the problem of Shivans' motives, so all interpretations of the few vague hints are equally valid as long.