It could happen. Really. As Trump’s campaign continues to drop in the polls, many pro-Trump anti-Hillary folks may go the the Libertarian ticket (Johnson and Weld) since, after all, both of them are former Republicans. If some PR bomb then explodes to damage Hillary (who knows these days?) and her numbers start to drop then some Democrats might go to Johnson since they sure won’t go to Trump.
An argument for people who do not relish four years of Hillary might also be what better way to smash the Washington gridlock than by putting a Libertarian in the White House.
So, if Trump continues to fall (say into the mid-twenties and then Hillary drops into the thirties the Libertarian candidate with 40% of the vote might just be elected president.