Will the country ever unite again, or are we destined to a divided America and will go the way of the Roman Empire?

I think Biden is too far right to unite the US ideologically, but as a democracy, we don't really need to be ideologically united as long as we're politically united and that's unlikely to falter any time soon. There are signs that our democratic institutions have been eroding over the past 20+ years, so it's possible that we'll lose our democracy and that could lead to some kind of insurgency or change of governance, but probably no actual secession except maybe from our overseas territories that harbor WMDs.