I think the argument would be that the Constitution provides for impeachment pursuant to a particular procedure. If you want to upset the decision of the people, then you need to follow that procedure. You can also see that adventurous prosecutors might use their power to conduct politically motivated criminal prosecutions of the President if that were not the rule. It’s not a slam dunk, but the Obamunists are probably putting their chips on that result in order to protect Hillary.
And if she loses, then they will not care so much if she’s indicted. They will hope that the rest of us won’t care so much if she’s not indicted.
The Constitution provides impeachment as the vehicle to remove someone from office. It it says nothing about providing immunity for anybody from any prosecution. If you can find an article or section that says otherwise please point it out to me.
If what you write is correct, then the President of the United States could murder someone on live TV and if the Congress refused to impeach, the president would be able to continue to rule as a known murder. I cannot fathom any Supreme Court that would agree with that assessment.