30 years ago a third party led the polls during the summer [1]. It all exploded after that, but I think that it shows that there is a possibility for a third to have an actual chance, although all the stars need to align. 30 years is a long time ago, but I think that the media landscape now is considerably easier for a third party than it was then (easier direct access to voters, national news intake is spread over more sources), and the current assumed candidates (biden & trump) unpopularity in the next presidential election seem to favor the possibility of a third party.
It might not be a probability, but I think it's a possibility.
I remember, but that wasn't really a third-party. That was just Ross Perot. I do think the two parties are slightly vulnerable to a strong enough, and rich enough, candidate surrounded by a cult of personality, but even in this case it would have been wiser had Perot simply challenged for the Republican nomination like Trump (another outsider) did 24 years later. He probably could have ousted Bush, and he really might have a won the presidency that way.
If en marche didn't win in france that would just have been one guy. Every party seems to be just one guy until it actually gains enough momentum an power to grow outside of that person and has to build an organization that survives them.
It might not be a probability, but I think it's a possibility.
1: https://www.nytimes.com/1992/06/11/us/the-1992-campaign-on-t...