Third parties will continue to thrive because of (1) popular dissatisfaction with what the Demopublicans and Republicrats have to offer and (2) the desire of some for power, passion for a cause, or the desire to make a name for oneself. Their fortunes will rise and fall depending on the perceived relevance of the major candidates to popular concerns. Looking ahead to 2020, assuming Trump is the GOP nominee, the best thing for minor parties would be for the Dems to nominate a boring, technocratic (and unlikeable) candidate (like Tim Kaine). The worst thing for minor parties would be for the Dems to nominate a fiery, passionate candidate with populist appeal (Bernie Sanders, Geoff Fieger).
You have an interesting interpretation of the word "thrive"
