Make up new names. Give them (pseudo) Greek names, Dilute trademark and genericise whatever it takes, but don’t undermine the meaning of well defined nouns for the sake of marketing on the coattails.
One of the basic features of language is that meanings of words change over time, including a given term retaining is original meaning but adding another one (did you think, for instance that all of the different meanings of the word "scale" were invented at the same time?). That being the case, using traditional meat terms for vegetable products that are similar to meat is perfectly legitimate.
Nobody who buys a veggie burger mistakenly thinks they are made of meat. In fact their knowledge they are not is usually a key reason they buy them
What if things get flipped in 20 years and lots of synthetic meats and other foodstuffs become the norm, would it be nice if the "natural" producers coopt their names and nouns to help sell their goods? for example, let's presume seitan is synthetic, it's well known for not being an animal product. But meat producers want in in their market and market "Natural Seitan"
I would say the main question here is if consumers would be mislead into thinking "Natural Seltan" had the same advantages over meat that artificial seltan has. I don't think they would. In fact, I think they would find that label confusing, and just stay away from it.
And let me add that if meat producers find sales under the traditional label have dropped so low they need to adopt that new one, then they would have already lost the war.
I'd probably find the name Natural Seitan misleading (i.e. mistakenly buy it thinking it was vegan), but calling it "Animal Seitan" would be perfectly acceptable/clear to me .
Gatekeeping what can be a burger or not is strange. Are you randomly ordering burgers and expecting a single recipe? What about those with chicken or fish in them, they also don't match the original recipe, which means you have to inform yourself about what's inside a given burger if you want a specific one anyway.
What about Coconut milk? Peanut butter? Do you think we should change their names too?
"Veggie burger" is a pretty well defined term at this point.
If you have a clump of meat, it's not a sausage. If it's brought into shape by some kind of "skin", (which nowadays often is artificial "intestine"), then it's a sausage.
So, I'd argue that the word sausage for example describes the form factor, more than the actual content.
I don't think the case is much different in the case of a burger - if it's clearly declared as a veggie burger, one better assume that there's no meat in it.
The distinction between meat and veggie burger is probably even less difficult than the distinction between a veal and a pork burger. You wouldn't want to have to use different names for meat products if they were made from a Kangaroo, or Ostrich, would you?
Who is undermining the meaning? Veggie burgers have been called that for decades. The meaning of burger as a cooked patty between two round pieces of bread is well established (chicken burger, falafel burger, whatever).
Make up new names. Give them (pseudo) Greek names, Dilute trademark and genericise whatever it takes, but don’t undermine the meaning of well defined nouns for the sake of marketing on the coattails.