I don't think our views on the purpose of impeachment are at any basic odds with each other.
We have very different views on the purpose. You have stated that the
main purpose of impeachment is removal from office. I reject that idea completely.
You have used your concept of the
main purpose as a basis for claiming that only current office holders can be constitutionally impeached. I reject that idea completely as well.
It's just that impeachment in my view applies to current officeholders, not ex-officials who are no longer in office. We have other procedures in our justice system using Federal civilian courts to deal with any crimes the official may have done while in office.
Then why bother with impeachment at all? The answer is... Because criminal or civil prosecution is not—and never has been—a substitute for impeachment. There are good reasons why the Framers included mechanisms for
both impeachment
and criminal/civil prosecutions in the Constitution—and for why they stipulated that impeachment does not bar additional prosecution in the courts for the same offense by the same individual.
That's because each have separate and distinct purposes and punishments that are not covered by the other. Impeachment can remove an individual from office and disqualify them from future participation in the political process; criminal and civil courts cannot do that. Civil and criminal courts can seize property; levy fines; and remove an individual from society and put them in prison—impeachment cannot do that. Unlike criminal conviction in a court, impeachment and its punishments are not subject to pardon at any time. "[The President] ... shall have power to grant reprieves and pardons for offenses against the United States,
except in cases of impeachment." One venue is never a substitute for the other.
While both venues are able to render punishments for certain acts of official misconduct and both may render punishments to the same individual for the same misconduct—they cannot render the same punishments. That's why both exist and why each have entirely separate constitutional provisions, rules, standards, and procedures; and are conducted by different branches of government.