And yet the two main parties here do feel the need to collude to exclude third other parties. Which demonstrates that they see third parties as a legitimate threat.

And third parties can get enough vote share to tip the outcome ("if only all those people hadn't thrown their votes away, my side would have won!"). Which means they're not the non-entities that theory suggests they are.

And parties aren't static, but have to adjust to match the electorate. There isn't a static steady-state to eventually reach.

And if you've seen discussions about the Democrat party in the US being a "big tent" party that's hamstrung by needing to appease moderates or the Republican party needing to kick out various extremists to gain legitimacy (why yes, most of the discussions I see do come from people on what we call the "left" here, how could you tell?), they sound like there's something similar going on to what I see in discussions about countries with proportional / parliamentary systems having to form coalitions post-election. Ie there's the same sort of coalition-building going on, it just happens before rather than after and isn't explicitly made legible in the labels candidates use for themselves.