Game out this scenario:
Republicans continue to refuse to vigorously investigate trump, continue to do stupid and wildly unpopular legislative shit like AHCA, and get trounced in the 2018 elections.
Democrats, now in charge, launch separate impeachment proceedings against Trump (treason) and Pence (conspiracy). Impeach them both, simultaneously, elevating the third-in-line-of-succession to the Presidency.
Speaker Nancy Pelosi, to serve out the remainder of the term until a full election in 2020.