The very idea that voting rights in general should be reserved to a specific class of person is inherently undemocratic and therefore any system that restricts voting to a specific class in general is no longer a democracy.
What you are talking about is an aristocracy, meritocracy, or other.
It would protect the country from the whims of the unintelligent who have no idea what is good for the nation.
A country run by the learned has a higher chance at being effective than a country governed by all. If the intelligent run the country, then the chances of a democratic mistake go down, because they are intelligent.