I also find "carnist" to be sort of a try-hard word in regular use, but the other part that SHOULD be said is that choosing to use it is useful for perspective switching. Typically being vegan or vegetarian is a minority position so the language in general basically normalizes omni eating habits. "Carnist" as a turn of phrase makes it possible to shift "normal" to more closely match veganism.

Edit: You basically said this deeper in the thread, oops. Still I'll leave it because maybe more people will see it.