Roman empire is not a good analogy, Caesar in effect made himself Emperor (or Caesar), when it had been ruled by the senate. It was hundreds of years before it finally expired (the Eastern Empire being last after it split).
I don't think Trump is really that big a deal. McCarthy caused untold damage for a brief period of time, was rejected, got dejected and died shortly thereafter. I don't think Trump's spawn can carry on without him, they've never had to do anything to get ahead except hang onto T's coat tails. He's going to huff and puff and blow himself up - this is the worst that could have happened to him at this moment losing him most Rs in Congress, with losing GA on his watch too. He's a busted flush.
The R's were written off in 2008 when Obama swept to power and the D's took both houses ... that lasted all of 2 years.