If his punishment is stopped by his no longer being in office. What stops an impeached president from stalling his trial for days, weeks or even months, waiting for the end of his term in office to take place prior to their convicting him. Thus leaving no punishment congress could impose on him.There is no impeachment for a former president over what some addled opposition party members feel he did while President.
And along the same lines what stops a Senate, say I dunno, Majority Leader, from refusing to schedule Senate sessions until after the impeached is no longer in office.