That's my big problem with the anti-third party logic: it's just an ouroboros. "Don't vote for a third-party, because they don't have enough votes to win." While that may be superficially true, if everyone
always buys into that tautology, then those parties will
never grow to be viable. So voting for Johnson, or Stein, or anyone else outside the Dominant Binary, isn't necessarily a "protest vote" (though I think people should be free to vote a candidate purely in protest, however petulant that might seem); indeed, for a lot of people, it's a "momentum vote"—casting your lot in for the "maybe not now, but maybe next time" idea—because, with somewhat historical levels of dissatisfaction on both sides, there really is a fairly unique opportunity to get some balls rolling, instead of falling back into two-party inertia.
Or, to answer the tautology with its obverse: if we never break the cycle, we'll never break the cycle.