I think it would work if it wasn't something that directly impacted the city, more like destroying a fallen base, that could have experimental tech, and if the fallen are able to repel the attack, then said tech could allow them to create your hypothetical fallen shock trooper, or if the base is destroyed, the city could use the tech, and perhaps new weapons are introduced.
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The scenario I used was solely for examples sake. Any situation, or variation of the scenario I provided could work.