I think if there were more than just first past the post elections where pluralities can win, you are correct, I think, to assert that the two current parties are likely to remain in the present duopoly.

However, institute another level of reform, like some form of irv, or multiple round elections designed to maximum (though not necessarily ensure) 50+% victors, or ultimately, some form of representative proporationality, and this would change quite quickly.

The Hun is always either at your throat or at your feet. Winston Churchill