Public opinion polls have been notoriously inaccurate in the third year of a Presidential term in who would be the Presidential nominees of major parties the following year.
In 2003, Vermont Governor Howard Dean was the front runner for the Democratic Presidential nomination, but John Kerry ended up as the nominee in 2004.
In 2007, New York Senator Hillary Clinton was the front runner for the Democratic Presidential nomination, but Barack Obama ended up as the nominee in 2008.
In 2007, former NYC Mayor Rudy Giuliani was the front runner for the Republican Presidential nomination, but John McCain ended up as the nominee in 2008.
In 2011, Herman Cain was the front runner for the Republican Presidential nomination, but Mitt Romney ended up as the nominee in 2012.
In 2015, former Florida Governor Jeb Bush was the front runner for the Republican Presidential nomination, but Donald Trump ended up as the nominee in 2016.
Beyond these examples, in the third year of many Presidential terms, who could have known that the next President would be someone not seriously considered at that time to have a chance to be elected.
Witness John F. Kennedy in 1959; Richard Nixon in 1967; Jimmy Carter in 1975; Ronald Reagan in 1979; Bill Clinton in 1991; Barack Obama in 2007; and Donald Trump in 2015.
Also add the following: Abraham Lincoln in 1859; Woodrow Wilson in 1911; Franklin D. Roosevelt in 1931; and Dwight D. Eisenhower in 1951.
So, to assume that Joe Biden, currently ahead in all polls for the Democratic Presidential nomination in 2020, will be the nominee is quite a gamble, based upon history.