The US does not have successful third party runs because of the uncertainty and difficulty of getting onto the ballot, the possibility of getting onto enough ballots that you will harm your allies but not enough ballots to win, and the fact that the electoral college means you can't rely on running up the vote in your home regions in order to get yourself into the running on a national level. It's not FPTP that prevents third party presidential runs in the US, but state electoral administration, state primaries, and the electoral college.
In any case, the effect is the same. It doesn't matter why the US only has two major candidates at this presidential election and the last presidential election and the next presidential election. It just matters that the US only has two major candidates at this presidential election and the last presidential election and the next presidential election. Abstention is nothing more than half a vote against the candidate closest to you. It's not a motivator for an extra candidate to run next time.
In this reading, the National Popular Vote Interstate Compact will probably have a bigger consequence than Ranked Choice Voting, because it will allow candidates to run up the vote in home regions, which will create new paths to the White House. Ranked Choice Voting just allows voters of the least favored candidate to make a protest and then come home. If you had had Bernie, Biden and Trump running at the last election with RCV, then Biden would have won if Bernie voters had preferenced him, and Trump would have won if they didn't. There's no actual alternative in which Bernie wins. So why would you stand if you're guaranteed to lose? It costs money and burns bridges. The extra candidates want a lower hurdle to winning (for instance, 38% in a field of four), not a higher hurdle (like 50%+1 in a field of four).
The consequences are different at the level of an individual district, because a member of Congress is a very different role than a national president.