Trump's followers and Senate Republicans are mistaken.
There is nothing in our Constitution that forbids the impeachment trial after the President has left office, bearing in mind that Trump was impeached for inciting an insurrection while President.
To be frank, the possibility certainly exists the Framers never thought of it. They never thought that Americans would elect a President as bad as Trump.
Trump was elected by virtue of several strange factors that will probably never happen again. The Democrats nominated the worst possible candidate, a candidate with a ton of baggage and facing possible indictment by the DOJ for her carelessness of state secrets. Voter apathy, 90 million voters stayed home on Election Day, and another ten million wasted their vote on third party candidates "in protest." Putin and Wikileaks combined to help Trump get elected.
Nevertheless, Trump was a minority President. He was elected by 27% of the electorate. His unpopularity throughout his Presidency reflects that fact.
However, once elected, Trump became the darling of extremists, white supremacists, neo-Nazis, current and former members of the KKK, the intellectually challenged, anarchists, and -- believe it or not! -- the religious right. The current group of malcontents like the Proud Boys and Oath Keepers are a reflection of that kind of support.
His fan club became a cult. They created fantasies about him and are completely clueless as to what Trump actually does and says. They get their information from far right commentators on Fox and other far right sources while ignoring the real news media because it reveals the reality of Trump. They make every attempt to avoid that reality. Trump's cult prefers their fantasies.
Unfortunately, the extremists in Trump's cult have not gone away, and they amassed 74 million votes in the last election, a force to be reckoned with. McConnell, Thune, Cheney, Cassidy, Burr, Romney, Toomey, and others are trying to steer their party away from the extremists, but it is an uphill battle. While losing the House, Senate, and the Presidency under Trump's stewardship, his followers remain loyal to him, not the Republican Party, indicating again the cult-like nature of Trump's fan club as opposed to a political alliance.
At some point Republicans have to grow a brain.
No doubt about it, the Framers never envisioned a man like Trump becoming President.