If democracies don't allow idiots to vote, that creates a disenfranchised class that does not accept the government as legitimate. An important role of democracy is to stave off class conflict, arguably just as important or moreso than having a government that serves the people efficiently.

A proper long-term solution is to have governments invest in critical thinking skills and education, but of course, it's a catch-22 situation.

Yeah, the problem is coming up with a test that actually identifies who’s “informed” without biasing the electorate unfairly.