They, too, should reconsider.
First: Trump could very well win. The polls before Brexit leaned slightly towards its success, but that depended on how you read them: almost as many polls showed it headed for defeat as success. Brexit took Europe by surprise. It's part of a wave of populist nationalism that we can recognize in many countries. Trump, too, is a part of the popular nationalism movement. Not only that, but there's still 3 weeks of election to go. If you believe Sam Wang, this has been an extremely stable election, and it's unlikely to tip. But if you believe Nate Silver, this has been one of the most volatile elections since we've begun tracking it, and the two previous big swings in opinion took less than a week each.
Second: there will be no getting around the tens of millions of people who will cast a vote, in effect, for American Fascism. If Trump loses, he'll create his own cable news network, which will have a built-in audience of millions. That network will drag Fox News further to the right. Trump will spend four years running a campaign against the legitimacy of the 2016 election, and will probably have little trouble capturing the nomination in 2020.
Remember: the GOP may not have believe Trump to be an existential threat to democracy in 2015, but they were sure as shit aware that Trump was a grave threat to their chances at re-taking the White House. His was a joke campaign. And yet he took the nomination from the party establishment, almost effortlessly. He will do it again. The forces aligned against him in the GOP proved themselves impotent.