Well, presidential systems like the US has mostly don't remain democratic for that long, we were lucky to have the spoils system moderating ideological conflict

Well, presidential systems like the US has mostly don't remain democratic for that long, we were lucky to have the spoils system moderating ideological conflict for most of our history and a period of unusually low polarization in the time since it ended. We've been seeing round of constitutional hardball like regular use of filibusters on every bill by the opposition that would have been unthinkable in 2000. But on the other hand we don't have senators seriously trying to kill each other on the senate floor like we did before the actual Civil War so as pessimistic as I am I'd be very surprised if things got as far as a civil war within the next decade. I wouldn't rule out a serious constitutional crisis though. EDIT: I'd highly recommend How Democracies Die, The Revolt of the Public, and Why We're Polarized for understanding the current circumstances.