The 2nd Amendment does a fantastic job offering (some) protection against an Alexei Navalny-kind of situation (i.e. protecting an otherwise helpless citizen from an authoritarian government that's out to get him). But I'll never believe the armed populace you're thinking of poses a real obstacle to full-blown tyranny. Ben Shapiro once told Piers Morgan the Holocaust would've never happened if Europe's jews hadn't been disarmed, but he ignored this disarmement and the persecution (in its early stages) were indifferently or approvingly observed by an armed populace who decided to do nothing. And that's not even to mention the well-armed and well-trained uniformed services which government has at its disposal.
Many believe the 2nd Amendment is outdated, a relic from a bygone area. They're wrong, of course – except in one regard: The threat to democracy which the Founding Fathers had in mind is clearly modelled after a monarchy, the only threat they knew. The 2nd Amendment can only function as its proponents say it does as long as the tyrannical government isn't supported by the people. What happens if the armed populace is indifferent towards or supportive of tyrannical policies? I'm afraid a little glimpse of that could be caught when the (theoretical) prospect of an armed populace's opposition did nothing to prevent Jim Crow laws, the Bush administration's use of torture or Obama's droning of even US citizens.
Personally, I think the right to self defence needs to be given a material underpinning in the guise of a right to bear arms. In that respect, the 2nd Amendment is a good thing. But I just see no evidence for its alleged role as a protector of freedom and democracy in general.
