The claim is that veganism is unsustainable and will die off
Carnists claim that veganism will be so popular that it will overtake the world and everyone will be forced to be vegan. But it now is so unpopular it will die out. Both can't be true.
The only citation for this claim is something vague like people supposedly leaving veganism in high volume with no subsequent citation or some sort of incredulity that relies on brain-in-a-vat levels of solipsism that everyone must be lying about being vegan.
The number of veggie burgers, vegan cheeses, etc. that have shown up in grocery stores has exploded. So this is kind of difficult to believe given that veganism is on the rise.
Also, consider that any plans to create sustainable colonies will probably need to be vegan as well. This sure seems like veganism is the path forward and that carnism is going to be a relic of the past.