That is obviously a factor too, but the fact remains that there have been plenty of third party candidates who have been on the ballot in all fifty (or almost all) who have struggled to get more than 1 or 2% on election day. If all minor parties got ballot access in all 50 states, then I can't see that changing majorly. If, on the other hand, voting operated on a more preferential basis, then people would not be afraid to put a Libertarian or a Green first in case the candidate they wanted the least won. Ballot access would likely only become an issue for the fifth, sixth, and seventh largest parties, and even then, the increased appetite for third party politics, along with the stronger position of third and fourth parties who are inclined to support reform ballot access, might boost their efforts to get it in many states they did not already.
The preferential system is also open to manipulation, we had Group Voting for the Senate and a dude known as 'The Preference Whisperer' organised to get all the micro parties (as opposed to minor parties like the Greens) to preference each other first. The result was that about 30 micro parties between them gathered up enough votes that when they were all heaped into a preference pile the Motoring Enthusiast Party got a Senator elected in Victoria on 0.51% of the primary vote. Would that be a result Americans would be happy with?
Preferential voting work better IMO when its used to generate a 50%+1 winner. The big parties must make deals and promises to minor parties to get their preference support which (in theory) the wants of the most voters get realised.
As for the US, I think the double whammy of the Electoral College majority in a state swinging the whole state makes it tough for 3rd parties in the US. Even if the Libertarians won an electoral college vote all the EC votes in that state would go to the Reps or Dems. That said a good showing from a 3rd party would put the frighteners on the 2 majors, but they're just as likely to make it tougher for the 3rd party as to make an effort to court these voters.