Didn't Montesquieu argue that democracy is essentially unstable and will devolve into tyranny in short order?
Not quite. Montesquieu's biggest contribution to political philosophy was the notion of separation of powers, based on his conclusions about the unhealthiness of concentration of power and authority in one individual or institution.
The answer he came up with was very different to British style, post-1689 constitutional monarchy. It was all a bit radical and French - which is why we Brits have always been suspicious of it, a view many here considered vindicated after the bloodbath of the French Revolution. But the US Constitution was built on Montesquieu's separation of powers concept and no-one could argue that it hasn't been incredibly robust or successful over the last 250 years (the Civil War period notwithstanding).
In my view the biggest issue facing the US right now isn’t Trump, or BLM, or neo-Nazis, or even Covid, but a widespread breakdown in respect for fundamentals like the rule of law and respect for democracy. Wherever you place the blame for that, and whether you argue all the problems of the world result from a conspiracy between big money and politicians in rigging the game against ordinary people, or of a grievance industry designed and run by Marxists to undermine the Constitution and bring down the state (what colour is your tinfoil hat?) the US definitely has a fundamental problem where each side doesn't trust the other that they will respect the electoral process or the operation of the justice system - indeed we are now in a situation where neither side might accept the coming election result if it doesn't go their preferred way. Democracy depends on the losers accepting that they they lost, on the proviso that they will get another chance to make the case in the next electoral cycle. In a society where there are more guns than people, any move away from that basic principle could quickly get out of hand.
In that context you'd think Biden's 'Everyone calm down', 'Let's make America normal again' pitch would get widespread traction, but he's clearly physically and mentally fragile, and the people around him aren't doing him any favours; indeed, US electorate would be fools to believe that the rioters and cancellers would consider their work done and simply fade away if Biden won the election.
I don't know what the answer is but I have faith to some extent in the American system. Separation of powers as well as the US's federal structure was designed specifically to prevent a notional Trump from wielding untrammeled authority and whatever the eventual ideological make-up of the Supreme Court, I would have faith that justices of whatever stripe would uphold the central tenets of the US Constitution.
Maybe that's a naive view but the composition of the Supreme Court has swung both ways over the country's short history without the doors falling off, and I have the view that they won't fall off this time.