It makes TOTALLY PERFECT sense to say that denying the vote to non-veterans would be the end of democracy. AS I recall, this was the law in Sparta, so it is not new.
Heinlein often included speculation of this sort of thing in hos books,and I doubt if he proposed it as something we should do. But Heinlein was a naval officer and he lived in a time of endless war, so the speculation is a valid one.
The question whether we should be an Athens or a Sparta was a common one in the `1950's Those who chose Sparta tended to send their sons off to military school, th "make a man of him" sooner than the eligible age for enlistment, Sort of like teaching your kid to read at home before the first grade, but for militarists.