Fundamentally a democracy's promise is leaders represent people's voice, and in exchange people follow the leaders.
Breaking that promise (e.g. cutting off "dumb" people from the process) means they'll have to find non democratic ways to express themselves. If they're in overwhelming numbers the shortest path is a revolution, and if a gov can just weather a popular uprising, it's a dictatorship.
And if the people who are cut off from the process are in the weak minority, and the majority agrees that it's good and proper to cut them off, then there will neither be revolution, nor dictatorship. This is the case with the people we typically disenfranchise today: children, foreigners, criminals. I don't understand why you keep coming up with these completely irrelevant hypothetical scenarios.
I was taking simpler to discuss numbers, but if you don't like hypotheticals:
Looking at 2020 numbers
> children
24% of teens or younger
> foreigners
13%
> criminals
23% of the US population has a criminal record
That's at least 50% of the population. That's a lot for a "weak minority".