CGP Grey has excellent videos on this. Here's one that discusses the problems with the system the US uses now (in most states): https://youtu.be/s7tWHJfhiyo
Plenty of other governments have first past the post voting and viable 3rd parties. The only argument where this holds water is with executive elections (president, governor).
> The only argument where this holds water is with executive elections (president, governor)
If you concede this point then I don't think there's much more to argue about. Nobody argues about the many third parties in the US that just endorse some major party candidate for the chief executive positions. When they run their own candidates for these positions is when they become spoilers.
Executives are pretty much what we're talking about with RCV, yes. We may be paying too much attention to those, but that's a bit of a different problem.
