Banning votes from toddlers is not as clear cut a point as you make it look like.
As a thought experiment: imagine an extreme society made 15% of childless adults, 5% of young parents and 80% of toddlers.
Would it make sense/be fair if the 15% of childless adults could pass laws that remove voting rights for life from anyone that piss their pants in public whatever their age ?
You could end up in a situation where 20 years later 90% of the adults of the country have no voting rights. Finding a way (setting the 5% of parents as representatives ?) to mitigate these kind of issues is generally important, which is why there's no cut and dry "good" voter suppression, only compromises.
Your preoccupations seem to be centered on protecting the system from demagoguery and outside influences, which is a valid POV, but that can't be the only angle nor the central focus. Even if 80% of the population was provably dumb, you'll still need a system that takes their voice into account to avoid the country getting overthrown or become a dictatorship.
> universally
Honestly I don't like that word, and it removes a lot of nuance that is utterly needed for politics and ruling systems. There is almost nothing universal, especially when it comes to "good" and "bad".
> Even if 80% of the population was provably dumb, you'll still need a system that takes their voice into account to avoid the country getting overthrown or become a dictatorship.
I really don't see how it follows.
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.
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".
The weight of cognitively restricted people and non-citizens in the voting process is less and less a theoretical issue, and would merit a lot more discussions IMHO.
Countries like Japan or Korea are getting into demographic phases where elderlies account for about 30% of the whole population and their voting power is tremendous, but we probably have no idea how good or bad the result is, and just cutting their voting rights as they reach some level of impairment would also be a seriously dumb move IMHO.
And on the other side as the fertility rate plummets bringing in more foreigners is an obvious option. Except these foreigners might not want to give up a stronger citizenship (e.g. an EU passport is way more valuable than a Korean one) just to get voting rights in their resident countries, and their kids will have a stronger incentive to go abroad as soon as they can if the country makes their life harder yet.
Partly in reaction to that, Korea for instance gives voting rights to foreigners mostly by virtue of residency.
We're entering very tricky situations where there's more imbalance between the ones holding decision power and the ones bringing the most to the table, and there's just no simple solutions nor any direction that is straight "good" or "bad" or unthinkable.