Many people disagree with your assessment based on the following:
No president has ever been impeached after leaving office, but there is one legal precedent that may be important.
In 1876, Secretary of War William Belknap was investigated by the House for corruption. Just minutes before the House was set to vote on his impeachment, he raced to the White House and handed his resignation to President Ulysses Grant.
The House went ahead and impeached him anyway, and the Senate proceeded to have a trial. A majority voted to convict, but not the two-thirds required, so he was acquitted. The scholars in the second camp point to this example to bolster their argument that even after leaving office, a president could be convicted and barred from holding future federal office.
Could Trump sue to stop a Senate trial? He could try, but it would be a tough case to win, because the Constitution says the Senate shall have "the sole power to try all impeachments."