>"If there are only bad candidates the situation is more complex, but this is an edge case which only happens in thought experiments"
I think you and I have differing opinions about the candidates. What do you consider "good"? Has there even been a "good" candidate in the past 20, 30 years?
I am not from the US, so I have to draw my "Sorry, I don't know" card here, but I really cannot believe that there were only bad candidates - It isn't impossible, but without proof it seems highly unlikely, i.e. an edge case.
Extreme variations are more likely the smaller your sample size is, considering America has a two party system they have a very small sample size causing it to be prone to extreme variation. Examples: No women candidate, no candidate that is not a politician, no candidate that favors alternate economic models, etc etc. This makes no "good" candidates according to a particular individual's view highly likely.
I think you and I have differing opinions about the candidates. What do you consider "good"? Has there even been a "good" candidate in the past 20, 30 years?