Yeah, define what you call unlimited democracy.
It's pretty obvious. Majority rules for everything? Not really sure what you're after here.
What I can do is expound on why it's not sustainable.
When the scope and power of government is minimal, losing an election to your political opponents isn't a big deal. There's only so much damage they can do, relatively few ways they can impact your life. But as that changes, when limits on government power are removed or diminished, losing an election can mean major changes to society.
Health care is a great example. As government gains more and more power control over health care, health care changes from a personal matter, to a political concern. An election can literally be a referendum on whether grandma lives or dies.
The point here is that the more power government has, the more the majority can force its will on the minority, the more painful losing an election becomes. Eventually things get so bad, the minority refuses to accept the results of an election. That's what we've seen in the last two elections.
When half the nation refuses to accept the authority of the sitting government, it becomes ungovernable. We're very close to that now.