Int he very early days we had FPTP just as we do now. So the fact that america has had primary parties switch 8 times, and other FPTP elections outside of the usa see switches all the time too, it disproves the fallacy that FPTP prevents third party from winning, we know for a fact it doesnt.
So that means the reason we dont see third parties win in recent history is clearly for a reason OTHER than FPTP voting, namely, the fantasy that it is a 2 party system when it isnt, or the fantasy that FPTP will guarantee a 2 party system when it doesn't. The only thing that keeps a 2 party system alive is the conspiracy theory that FPTP forces a 2 party system.