Wrong-doing must always be prosecuted, even if the perpetrator got away. Otherwise, we set the precedent that you can engage in all kinds of malfeasance while in office, and that you can get off scot free when you leave office.
That’s my line of thought as well. He needs to be punished in some form or fashion by the justice system to deter others from this kind of activity.
Impeachment is not to be used for prosecution. It is for the removal of an official only. After removal, without the shroud of power over him, an elected official is fully subject to prosecution of any crimes.