In terms of what's coming in 2020: from the purely polling point of view, polls are suggesting that Biden currently enjoys a 10-point lead nationally, and something like a 5-point lead in several key states. At this point it is almost certain, at the very least, that he'll win the popular vote by a margin greater than Clinton enjoyed in 2016. Whether he'll win all the states he needs is less certain.
One other point is that, taking the polling and its interpretation as accurate, Trump currently has around a 12% chance of winning. 12% is not insignificant: it's about the same as the chances of tossing three tails in a row. It would therefore be a surprise, but hardly a shock, if he won from this point of view.
In essence, you're confusing two types of mistake. It could be, of course, that the analysis I've mentioned above is completely wrong, and Trump's chances of winning are much higher. Or it could be that they are right, but that he wins anyway, because unlikely events are not impossible ones.