I'm failing to see the logic here. Against the omicrons, being sundered would indeed pose a problem, especially if they could fell even unsundered stars, albeit at a great cost in the case of the Dragonstar. The only reason sundered Etheirys might be spared from them is it would not even be regarded as worth the effort. The Omicrons were a full race, but they were also one that had digitised their entire existences (hence, I consider automatic inferences in the case of Omega, a construct's of theirs, to be referencing dynamis, to be purely speculative in nature) and had lost a great deal in the process. How might a 1/14th Source fare against a full blown invasion of their kind? 2/14ths? Etc. Most likely a non-issue with them because they gave up to apathy, but let's say another star did. Having a few heroes shielded by boatloads of plot devices and plot armour does not mean the rest of the aetherically divided Source would be able to handle an invasion from a hostile star at full strength. Were it not for that, the Endsinger would've devoured the souls on the star whole through the Final Days' repeat, so it's not like the sundered (albeit 7 times rejoined) Source itself is insulated from future risk, even if she is now gone.
And frankly, who cares whether he "buys" it or not? It is their people and star which were aetherically divided 14 times over. Not his. All he knows is that his people suffered an unfortunate fate before another star. He knows nothing of the future of the Source, of whether it too might succumb one day to a similar dead end to all those other stars. In the end, the only reason Hades and the other ancients were blind to the fate of the other stars is because this knowledge was withheld from them, thanks to Hermes as well as Venat and the time travel contrivances. Midgardsormr is none the wiser to it. Bit bizarre to hold it against them if the knowledge was never shared with them. Meanwhile, while some of his children got along with man, we know man's greed led to another getting locked in a brutal conflict with them, so it wasn't inevitable that this star's fate could've ended much the same as those others which turned hostile to the dragons, particularly if Nidhogg gained the upper hand. Again, you can thank the protagonists and all their plot armour for it not going that way.
I'll grant that the "wrong" in this case depends on whose perspective we're viewing this from. I take it you mean the third round of sacrifices. As for the ancients who had the Sundering sprung on them with precious little understanding of what was truly going on behind the scenes...? From their view, it is sufficient that they are reversing a great injustice inflicted upon them. Even if Venat had some kind of moral qualms with the third round of sacrifice (and if it was just creations like those coming out of Elpis I can't say I'd share them), they still might not consider that good enough reason to sunder the star. In the end the entire plot is now mired in a bog of extreme vagueness so unfortunately we're just filling in holes through speculation, which they might address later on - or they might not. Hades tired, set a test for the sundered whereby he'd pass the baton, and honoured his word.