When was the last time the pv loser got this low of a share of the vote and still won?
No PV loser/EV winner has ever gotten this low share of the vote.
When one starts with a 55% unfavorability rate, one has much credibility to develop and many bridges to build. An even 2% shift in popular vote nationwide causes him or his successor to lose Michigan, Pennsylvania, and Florida in 2020. Even shifts are rare except in nearly-even races.
People whose sensibilities he insulted in 2016 will have a hard time voting for him in 2020... and many who voted for him will have to decide that he served their needs. So he promised jobs that did not appear or brought those jobs about by degrading the pay and working conditions of those already at work. If one still has a job but is living worse while economic elites live in opulent splendor, then one might see only a raw deal.
Donald Trump will have to betray the desires that he fostered among many workers because he made promises that cannot be delivered together. Add to that, most of those who got shut out of the political process for four years will want back in, and they will be strongly motivated if they are not shut down altogether.
Is there a new JFK/TR/Obama for the Democrats? Such a young nominee would better understand the economic and political values of younger voters while not being beholden to special interests of aging groups within the Democratic party. Remember: by 2020 all Boomers (if you use Howe and Strauss/ divide between 1960 and 1961 between Boom and X -- Barack Obama is no Boomer; he acts much more like such Presidents as John Adams, Cleveland, Truman, and Eisenhower than like Clinton or Dubya) will be 60 or older at the end of the year. Elderly politicians can govern, but they need younger supporters for the bulk of their votes. Hillary Clinton got tepid support from the youngest voters and may have lost for that.
Minorities? It is easy to say that the Republicans will do better among them because they couldn't do worse than in 2016 while running a frankly racist campaign -- but it takes a long time to undo the damaging perception.
This is before people get to see his political persona fully sink in without the pressure to moderate to seem tolerable to moderates, and before he gets culpability or praise for his economic, military, or diplomatic policies. He is about as reckless as Obama is cautious, and such bodes ill.