Carnists spend their whole life eating meat. How is it that they suddenly have some burden of proof? Shouldn't continuation be the default and change require justification?
This is an argument that what you've been doing is morally correct merely because you've been doing it. So therefore your argument for hurting animals can't rest on this argument. And this sort of makes sense when we just even think about it for a second. I mean, if you saw someone stuffing baby animals into shredders, would you say that it's a fair argument that "Oh, well I've been doing this for so long you would really have to convince me that I'm wrong." We just wouldn't accept that they are already doing it, therefore this makes it more ethical than if they were just getting started, so they should continue doing it.
Just because someone has stolen cars their whole life doesn't mean that all of the sudden not stealing cars requires some kind of justification, and that theft is now the moral default. It might be the "default" behavior of that person, but that doesn't mean that it somehow becomes the moral default.
Does that also mean that if someone has never provided evidence for any of their claims, that somehow now not providing evidence is now the default and that their claims, unfalsifiable or otherwise, should just be blanketly accepted? Why then should we accept this when someone engages in special pleading for the rights of certain species or humans over "food" animals? If someone has a history of being illogical, do we just say that "non-logic" is the default? This is just not an internally consistent or coherent position.