On your GC analogy: the principle objection I have is that the barriers Runciman describes don't remove the poor / uneducated / old, they disenfranchise them.
What you're describing sounds far more like eugenics, which has proved quite problematic. In the context here, one additional problem is that removing the young eventually removes the population, as they're where old people come from (eventually).
I'll also push back on the poor being unproductive. Often the poor are productive but have limited compensation negotiating power, and as contrasted with, say, the wealthy inheritor class living off rentier income and/or trust funds, probably compare quite favourably in actual economic productivity.
Those lacking formal education may also be quite productive or capable (another podcast I'm listening to just mentioned Michael Faraday, whose formal education ended early; he was apprenticed to a bookbindary by age 14), and the young are typically at the early end of both their productivity and wage-bargaining curve. So a few of your premises may require revisiting.
Again, what Runciman is describing isn't a culling of portions of the population, but of deciding whose voices are heard and/or influential politically.
I'm also thinking of the long history of the spread of the electoral franchise over time, in multiple countries. Typically from propertied males (often restricted by ethnic or cultural origin) which dates back to the original democracy in Athens, then typically more inclusive classes of cultural-majority males, women, and finally various ethnic / cultural minorities. Even in what's generally considered an advanced democracy, the United States, there are current attempts to exclude specific groups from the franchise, either by law (e.g., convicted felons, even after their sentence has been served), or through other restrictions (limited polling places and times, restictions on vote-by-mail or other remote / early voting options, registration limitations and requirements, etc., etc.).