Tim,

Let's assume that the convergent utility function supported by natural selection is that of a pure survival machine (although it's difficult to parse this, since the entities you're talking about seem indifferent to completely replacing all of their distinguishing features), stripped of any non-survival values of the entity's ancestors. In other words, there's no substantive difference between the survival-oriented alien invaders and human-built survival machines, so why bother to pre-emptively institute the outcome of invasion? Instead we could pursue what we conclude, on reflection is good, trading off between consumption and investment (including investments in expansion and defense) so as to maximize utility. If a modestly increased risk of destruction by aliens is compensated for by much greater achievement of our aims, why should we instead abandon our aims in order to create exactly the outcome we supposedly fear?