Where does that idea come from?
How do you figure that Johnson or another 3rd party candidate is a non-factor?
Having more choices doesn't steal votes from the major party candidates, it gives us(the people) a chance to throw what little weight we have around. I've been studying politics since I was twelve and there's a pattern of dissatisfaction amongst the populace which indicates a tipping point. For historical reference Lincoln was a 3rd party candidate and it's happened 4 times previously in the history of the country, a rare occurrence that only happens under the right political climate. Like what we are seeing now with the current presidential election.
The current major party nominees are completely out of touch with solutions for our(the people's) lives and a large portion of the population is, in actuality, ideologically, in the middle of the extremist platforms presented by the Democratic and Republican parties. Which means that right now is a great time to elect a president that actually reflects that. Johnson(Libertarian), Keniston(VPA), and Stein(Green) are all of that ilk. Links for their sites are posted below.
Mathematically speaking, odds wise, it's a long shot even in base numbers. Here it goes: If each of the leading 3rd parties (Libertarian, VPA, and Green) get 30 million votes, that's 90 million that the Democrats and Republicans don't get. If you split what's left of the registered voters (assuming that they all vote) each would receive about 35 million votes and the house would have to decide from the top 3. As a bonus, in 2020 we would have 5 major parties on equal footing with respect to funding, ballot access, debates, and media coverage. Did I miss anything?
Numbers are rounded from http://www.statisticbrain.com/voting-statistics/
Websites for 3rd Party Candidates: https://www.johnsonweld.com/