I don’t disagree with that specific criticism, and Heinlein could be as cranky as anyone. That doesn’t mean that the sound-bite version is actually wrong, though, as long as it’s accepted as a rather short and pithy saying rather than literal gospel handed down from on high.
I sense a “No true Scotsman” argument under the surface here. If a government is a democracy on the surface but in fact is corrupted by kleptocrats, political machines, or anti-democratic social movements, then the government really isn’t a “true” democracy and it’s ok if it fails.
No one said “it’s OK if it fails”. Sure, Greece is a Democracy. And no, it didnt fail. It came close. It’s the closest to the Heinlein quote and even so, it only had issues as the government found they could borrow like crazy from other EU nations.
Venezuela & Zimbabwe had some democracy but before the bad times started they were no longer democracies- the Autocrats were what caused the issue.
Just because the National Socialist German Workers’ Party has “Socialist” in it’s name doesnt mean it was so. Just because Hitler was elected (sorta) doesnt mean he ran a Democratic nation when he was in power.