The polls in 2016 were actually pretty accurate, at least a the national level. The final batch of them predicted that Clinton would beat Trump by about 2% and that s about what she beat him by.
What went wrong was political prognostication which missed Trump s unusual strength relative to Clinton in some midwestern states. Many commentators also didn t accurately factor in the October Surprise of the FBI reopening the Clinton email investigation. That ultimately did Clinton in but I think most political forecasters assumed that earlier Clinton leads, where she was more like 5-6% ahead of Trump, would hold.
The people who got it wrong actually weren t the national media so much as Trump supporters. All throughout the campaign, Trump and his supporters were saying that the polls were wrong and that he was immensely popular, or at least more popular than Clinton. They were all wrong. Trump was not only less popular than Clinton, but he was less popular (as measured by vote share) than any candidate since the 1990s. Trump supporters also got the outcome wrong in another way. Some of them were sure that Trump would win the popular vote, but felt that he might lose the electoral college because of Democratic strength there. Of course, this ended up being completely backwards.
In terms of looking forward to the 2020 race, I think that Trump supporters also need to look at the 2018 election. Throughout that election, the polls were showing us one thing consistently: that the Democrats were going to have a good, perhaps great, showing on election night. Trump supporters, and Turmp himself. kept giving us the same song and dance they are now: "The polls were all wrong in 2016. Trump is popular. Democrats are going to lose". Guess who turned out to be right in 2018? The professional pollsters who actually relied on data. The Trumpsters, who relied solely on their own wishes, turned out to be wrong. I think that Trump fans should realize that Trump s victory in 2016, and the failure of most political hands to predict it, was really a fluke and not a sign of systemic problems with the polls or some sort of hidden popularity on the part of Trump.