It's unlikely that the US will fall into chaos and political disarray so those are not really the problems people are talking about when they mention increasing income inequality and more polarized politics. And you're right that empires don't really fall in short time frames, it's a more drawn out process but when decaying institutions are combined with shocks to the system like global warming, famines, wars, pandemics, etc. then it becomes much harder to be optimistic about the status quo and the people in charge of managing these shocks.

Things are a lot more fragile and unstable than people realize so it's important to zoom out and look at all these issues in aggregate in order to formulate effective plans for revitalizing decaying democratic institutions so that we can properly deal with the inevitable shocks that cause the decline and collapse of nations.