Insofar as there's an argument here, its some galaxy-brain stuff about how, if deprived of left-wing activist energy, Labour will be even more centrist. Again, this sounds like a "you" problem. Underlying this is the idea that the Greens need Labour to be a cooperative partner in order to enact their policies. Which assumes of course that the current major party / minor party dynamic between them will remain. And I don't think that's a given. Green activists are pretty explicit that their goal isn't to work with Labour, but to eat them, and while I'm cautious about the speed at which this might happen, the current trajectory certainly suggests it is possible. Unattractive, arrogant whining like that discussed here only makes that more likely.
Major parties have fallen before. In 1919, the Liberal Party, which had been the progressive force in New Zealand politics for thirty years, was challenged by an upstart on its left. That party was Labour. The Liberals did some very familiar whining about "vote-splitting" - which meant more under FPP than it does now - but ended up being gutted by the departure of their progressive activist base. Just a few elections later, they were dead, rebranded as United. Two elections after that, their remnants had merged with Reform, their former professed enemies, to become National (which is what centrists always do: go right). Labour can avoid that fate, by not being crap. But again, that sounds like a "you" problem.