I don't actually have a problem with federalism as a default position on government authority, although I move further from it as time goes on. But occasionally it has been necessary for the federal government to overrule the state in order to protect the individual. I lament each time states make it necessary for Uncle Sam to step in as it reduces their independence, but we only find ourselves in these situations when states abuse their authority over their citizens.
The Romans indeed had a long list of components in their fall. To the reasons already mentioned I'd add communication limits, budgetary constraints, questionable military "reforms", and patronage off the top of my head, but it's been a while since I read Gibbon or Luttwak.
I'm disappointed but not surprised by Trump here. It pretty much forces me to hold my nose and vote against him.