> There will always only be Democrats and Republicans, and because the system is a duopoly,
Maybe, but people probably thought that about Democratic-Republicans and Federalists, or Democrats and Whigs. The subsequent realignments have so far been more gradual adaptations within the existing major parties, but a radical and suddenly realignment destroying (and replacing, either immediately or in short order) one of the majors is still possible.
> Dislodging the duopoly (that is, opening the electoral system to 3rd parties) will require proportional representation full stop.
Something like that, yes. (Not necessarily party-list PR, but electoral systems with greater proportionality than FPTP.)