I agree with you. There are 2 factors that would make 3rd parties competitive:
1) Money. By far the most important. Without money, you can't get your message out. Dems and Repubs have the fund raising apparatus and the large donors and superpacs (dark money) that spend big sums on behalf of their candidates and issues. Unless there is monumental reform in campaign financing (i.e., abrogating Citizens United) it's highly unlikely any 3rd party will have the resources to take on the 2 major parties.
2) Ballot access. Getting your candidate on the ballot requies navigating 50 different state requirement. Like signed petitions, and having won various percentages of the votes in previous elections. Rules which were enacted by major party legislators in order to maintain their dominance in elections.
If you can think of a way around thsese obstacles, I'm all ears.