In practice I don't think 85% of Americans would be unqualified based on those stats, those numbers would improve dramatically if the incentive were there (IE. know these things or don't vote).
>If you give people the obligations of living in a democracy (paying taxes, subjecting themselves to its laws, signing up for the draft, etc.) without actually letting them participate in that democracy, then it's not a democracy anymore.
I see your point but I don't consider democracy to be a binary condition, it exists in degrees- IE. voting directly for policies vs. electing representatives to do so, etc. If one were barred from democratic participation on a permanent basis for reasons they have no control over, IE. Race/Sex, then yes democratic principles have been violated. But if all they have to do is learn some basic facts about the history of the institution they are participating in, I don't see that as 'not letting' them vote. It's more 'if you really care about voting you'll do some basic due diligence first'.