This caught my eye, I don't know if it's been addressed further down.
First past the post/winner take all voting structurally causes third parties to either be irrelevant or ascend to take the position of one of the duopoly parties.
As a highly simplified example, suppose you had trump, Biden, and (to round out our cast of old guys) Bernie running in the general election. Also imagine that we live in a world where half the voters are liberal, and half are conservative. I think it's fair to say that conservatives are generally going to be most likely to vote for trump, and liberals are more likely to vote for Biden or Bernie. So despite the "two sides" being roughly equal In numbers, if a significant amount of the liberals vote for Bernie as a third party candidate, suddenly that vote is split between Biden and Bernie, and even if the overall political side had an advantage they'll likely loose.
This leads to voters who potentially align more with a smaller candidate strategically voting for the more popular candidate that only sorta aligns with their values.
In this environment third parties are dead, their only hope is to kill one of the current two parties and yoink their base
This type of thing is partially mitigated by parliamentary and coalition based systems, though the most feasible fix for the us is likely something like ranked choice voting
TL;DR: our voting system makes a two party system inevitable
