He lasts one term and decides to not try for another term.
Impeachment is a fantasy. Or rather, if 2018 goes the D's way in the House, he may be impeached because they are without reason or standards. But it takes 2/3's of the Senate to kick him out of office. Which will never happen based on what we know today. He lasts 4 years, decides it's not as fun as he thought it might be and step aside.
mspart