If von Neumanns were practical in the FSverse don't you think one of the pre-Ancient victims of the Shivans would've tried using them?
The answer to this question is much more interesting and intriguing than one might initially think. I think of Von Neumanns obviously as "living beings" that could be obeying darwinian principles and so on. In darwinian speek, what you are saying is that there are a lot of strange attractors that might be funnelling the landscape of genetic solutions towards something ultimately impractical against Shivaggedon. A lot of odd questions can arise though.
First off, is that even true? Is it true that *no* species has managed to survive against the Shivans in this way? According to FS2verse and BPverse, we have not sufficient knowledge of the galactic landscape to know the answer to this question. So, possibilities abound. It is possible that many species have survived this way. It is even possible that many of these Von Neumann species have competed against each other for their own survival. It is possible that some, a few or even a single one of such species actually dominates the galactic landscape.
Another completely different possibility is that there is very little competition between these Von Neumann life forms, since obtaining resources for their very energy-efficient economies might not actually interfere with any other Von Neumann life forms. However, in the landscape of possibilities, if enough variety is achieved, you would expect some of them being more aggressive than others.
Many have already thought about Shivans and Vishnans *as* Neumann probes, or we could even say these denominations are too simplistic to even begin to define these higher webs of complexity. Perhaps one of the first intelligent beings in the Galaxy ended up creating such probes, and when a very aggressive species emerged from darwinian pressures, they started competing with each other. One darwinian strategy that may have emerged from natural selection pressures was precisely the genocidal strategy, where a species genocides an entire living species when it begins its warfaring before
a new Neumann species is created by the organics.
Think of it as "Mass Effect done right".