The problem is that our system is winner take all with no possibility of coalition government. Such a setup will almost always produce a 2 party system. Doesn't matter if it was by design or not, the result of this type of electoral setup will almost always be 2 competing party's trying to get 50%+1 of the ballots cast.
Ballot access makes 3rd parties more difficult than does a winner take all system.
The 2 parties in power must submit zero for their candidates to appear on a ballot.
A 3rd party must submit, in a national campaign, a total of something like 1.3 million signatures, depending on the states and their terribly non-uniform requirements. This is not counting the legal hurdles that must be overcome just to qualify to submit the petitions. Then there are requirements about signatures, collectors, forms, etc.
IF 3rd parties were so innocuous, there would be no good reason to have these legal hurdles in place preventing others from joining the game.
More fair, probably, is for all slots on the ballot to be write-in slots. Period. That leaves it entirely up to the voter whom to vote for. Those who run solid campaigns will be known, and those who don’t won’t.