I just revised my post of yesterday to observe that it's difficult to predict the future.
Obviously the tendency is to project trends of the present into the future: in 1960 South Korea is a dependency of the US, in 2020 South Korea will be a dependency of the US; in 1950 the Red Chinese were a horde of indistinguishable people wearing Mao jackets; in 2020 the Chinese will continue to have no individuality and dress alike; in 1950 Japan makes cheap children's toys (still remember a metal airplane toy which made a noise when you pushed it along the floor; in 2020 Japan will still be behind the curve of technology.
Mr. Taleb of "Black Swan" fame has a theory of why we fail; a theory I forget the content of. It's possible we just err. Or it's possible we like the comfort of the known and dislike Rumsfeld's "unknown unknows".
It would be an error, I think, to assume that President Trump is doomed to be as unpopular on election day 2020 as he is today. Things may happen, or they may not.