It's like people don't remember that there was a president that called a press conference, spoke to the people, put on a jacket, got in a helicopter, and flew away from the White House forever.
Presidential elections happen on schedule. If Trump should leave that gives you Pence, if Pence should leave that will give you Ryan. And likely by that time it will be 2020 and you'll have those new elections but until then you'll be stuck with a Republican as president, like it or not.