An issue to keep in mind when even approaching topics
remotely associated with eugenics is how absolutely derailing they can become. Bull in a china shop, so to speak.
Among my own objections is that deliberate negative eugenics seems to me to be a case of evolutionary self-selection, that is, endogenous rather than exogenous evolutionary pressure, which ... seems particularly prone to various feedback effects. Self-selection is not entirely excludable ("mate selection" is a well-known example), but it seems strongly associated with nonfunctional-though-indicative features (the peacock's tail, moose antlers, stotting) in non-human populations. Numerous cases of pro-eugenic policies seem to be based on at best specious "fitness" indicia.
The question of disenfrachisement and its ill effects presupposes the larger question of what the purpose of democratic governance is, as well as what possible alternatives might exist.
Runciman's essay is interesting to me beyond the "poor / uneducated / young" aspect, in that it also addresses the option of sortition, which is a fairly fascinating concept I've been looking at for about a decade now, see "If You Can't Choose Wisely, Choose Randomly" <https://aeon.co/essays/if-you-can-t-choose-wisely-choose-ran...>. Runciman directly notes that this still tends overwhelmingly to favour representation of the poor and uneducated, which is probably non-ideal.
If we consider polity as the process of achieving collective decisionmaking, and see among the multiple goals of such systems not only effective and good decision outcomes, but social buy-in, fairness, broad consideration, and minimisation of self-serving corruption, then the arguments in favour of democratic processes (over oligopic or autocratic forms) seem more evident, though there are also failure modes of democratic forms. That many of those tend toward oligarchic or autocratic (electing a strong man, outsized influence of campaigns and/or media by a small high-wealth / high-power cohort, for example), bears reflection.
There's also the question and problem of effective representation. A member of the US House represents about 760,000 people. Over a two-year term of office, this means that each constituent can on average claim 1m 23s of their representative's time, assuming the representative is available 24/7/365. Congressional staff and household aggregation assist somewhat with this, but calling the circumstance "representation" seems a stretch. At the same time, increasing the size of the House by the 10x or 100x which might better suite effectual representation would make that body even more ungovernable than it already is. In other contexts, notably both military and corporate organisation, multiple spans of control, each typically addressing 4--20 or so individual units, is common. This of course leads to strongly hierarchical organisations at large scale, but these typically do have effective communications and command flows. Military hierarchies tend to be fairly stable over years and decades, whilst commercial organisations tend to see frequent (and often incomprehensible to those involved) reorganisations, shuffling those relationships.