Why do people think Trump winning isn't at least 50% probable? Herd mentality is very well ingrained and will overtake even rational individuals, and we know that rationality is less than 50% in our country (and outside too). Most of the people buy stuff they don't need on credit cards, they chug sugary sodas, and their news is through memes. And these are the educated and employed ones. Rationality has become a rare commodity. I would guess that the majority would look at a few video clips of the presidential election debates and vote according to anything but rational thought (have always voted this way, job is going well so country must be good, Trump is funny, etc.)
An incumbent has the advantage of name recognition, and in Trump's case, a lot of shenanigans that are possible to those willing to abuse power, ranging from lying blatantly with a guaranteed wide reach to abusing anything and everything to secure votes, whether pushing coal or pushing kids in cages.
I would give Dems the higher chance of winning if they were somehow united and had a simplified crystal clear message. They are far from united; the party prefers Biden and is scared of Bernie because they are too comfortable to rock the boat, and there are 100 messages from 10 candidates. Unless Dems go cold turkey and embrace the 21st century wholeheartedly, there is a good chance that the party that is clinging to the 19th century will win.