Radicals ruin everything.

The problem is that any third party that manages to eventually displace a member of the duopoly immediately replaces that party in the new duopoly.

Because the duopoly is a result of First Past the Post (FPTP) voting. As long as we use FPTP the duopoly will persist, just with different parties filling the two roles.

Anything short of switching away from FPTP for some form of Rank Choice is going to be a band-aid, mere temporary relief, and not even a very good one.