we are moving closer to a structurally codified duopoly

Not true. It just happens that in the last election we got a more duopolistic parliament than usual. There is no general trend however. Here is the list of all parliament makeups since confederation. You would have to go back to the 1958-1962 parliament for a two official party parliament and even then it was just that the CCF only had 8 MPs and so didn't have official party status. To find a parliament with legitimately only 2 parties, you have to go all the way back to the 1918-1921 parliament.

Our system consistently produces 3rd and 4th parties, and operates comfortably in minority governments.

To be 100% clear, I am not making a case against proportional representation. It is absolutely the right way to go. Electoral reform is absolutely essential and we should be thiniking out of the box to deepen and strengthen our democratic institutions, e.g., replacing the unelected senate with either a kind of citizen jury via sortition or straight up the Assembly of First Nations (now that would be something :) ) and definitely a massive increase in the number of MPs, to massively increase the winning coaltion (ratio W/S). There is zero reason why 19th century technological constraints (send a bunch of dudes to sit around in a big room) should define what 21st century mass democracy should look like.

What I'm pushing back against is not electoral reform. I'm pushing back on seeing Canadian politics through a US lens. Liberals especially loooove that lens because it means they get to use the craziness of the Poilievrites to blackmail everyone.