"Your rationale has a serious flaw.

It appears you are arguing that conviction can only occur if removal from office is still a valid remedy yet to occur. This is incorrect.

Similar to the criminal justice system, even if the prescribed sentencing cannot be carried out or is already underway, a conviction is still possible. Even if a judge where to sentence the convicted to time served, the conviction still stands.

Thus, any subsequent penalties that may have had conviction as a prerequisite can proceed.

Impeachment is no different. Precedent from John Quincy Adams and other non presidential impeachment cases confirm that impeachment can proceed after leaving office. Although removal from office is the best-known remedy for an impeachment case, it's not the only one."

^ I found this convincing as a response to his article.