If I go to one of the chain burger joints, it's because I want to grab something to eat on the run, not have a sit-down meal. Actually, I typically see more old folks eating there than I do families with small children. My biggest complaint is that they seem to have raised their prices a lot more than other eateries...tough to get out for under $10 anymore. Remember when McD's advertised change from your dollar?
If any of those places snuck a vegetarian burger into my order I didn't notice it. Nothing I'd ever order. I did once try some of the Beyond fake meat (was in the markdown bin at the supermarket). Once was enough, it wasn't very good.