This is factually incorrect, the president can be prosecuted for anything, he doesn't even have to commit a crime, but there's a special process for that called impeachment. This isn't new stuff and it has been understood to work this way for a couple hundred or so years, until very recently.
Even the majority opinion doesn't agree with you. Literally the only people I've heard make this argument are Trump's lawyers in this case and in fact during the impeachment proceedings they said the exact opposite. Impeachment is a separate process and the president doesn't need to be impeached to be charged with a crime.
The impeachment clause specifically lays out that impeachment doesn't inhibit criminal prosecution for the same acts. Hell, Trump's impeachment defense was essentially that he should be criminally prosecuted instead of impeachment.
