The right answer is : nobody knows that, and only the future will give us an answer. It's certainly not as unlikely as some people tend to believe.
But if you look back to past Third Party bids, you might consider that they suffered more from bad luck and/or bad candidates than by structural disadvantages. Perot stupidly dropped out and lost 20 points because of that. Anderson suffered from Reagan's late surge. Wallace's segregationist stances prevented him to gain ground outside the South. In 1912, hadn't the GOP been in power for 16 years, it's possible to imagine Wilson performing less well and thus Roosevelt having stood a chance. Also, hadn't populists taken over the democratic party in 1896, it is also possible to imagine a scenario with a Populist party gaining ground in the country and finally acceeding to Presidency.
To put it clearly : it's not because something has been in a way for a very long time that it will always be in that way. Also look at
this article from Nate Silver about the eventuality of a Third Party bid in 2012 and its probability to succeed.
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