That's an interesting point. I was vegetarian/vegan for a long time (decades). Err, I say it that way because it is easier for people to get a vague understanding of how I ate, but in reality I just really like vegetables and like cooking with them. There was a comedy sketch on the radio in Canada one time about someone who would order a veggie burger with bacon and I was kind of like that (bacon is delicious).
A lot of people who are not vegetarian get the idea that the ideal vegetarian diet is basically the same as a normal North American diet, but without meat. The first questions people ask are, "What do you do about protein?", or "You must eat a lot of tofu, don't you?" or "How much salad do you eat?". They can't really comprehend a diet without something in the "meat category". In reality, my experience was that most long term vegetarians just eat a completely different diet. You aren't looking for "meat substitutes" because it doesn't occur to you that you are missing something. I rarely ate salad (don't really like it), or tofu (unless I made it myself -- in North America tofu sits in the store for weeks and becomes bitter and horrible), or "meat substitutes". Just legumes and rice or other combinations in curries and stews, etc, etc.
My wife eats meat/fish and so I am no longer a "bacon-eating vegetarian" but there is still a place where I would use a "meat substitute" -- basically where you want a filler and you don't want a meat flavour to dominate. "Hamburgers" can fall into that category. Consider most fast food burgers -- the flavour you get is dominated by the condiments and other ingredients. I really enjoy a veggie burger with a really nice slice of pickle and a great bun, good mustard, etc. If I'm going to eat beef, I vastly prefer to have really good beef and virtually no condiments. I want beef to dominate. But there are many times (most if I'm honest) where I want the flavours of the vegetables and bread to dominate. In that case a veggie burger is actually better than beef (hence the veggie burger with bacon).
Probably not that many people consciously agree with my idea that a vegetable dominant burger is desirable, but in fact there are tons of people (the majority, I think) who really like burgers very little meat flavour (i.e. the people eating big macs, etc) And if you give them a really good beef burger, they will drown it in sauce so you can no longer taste the beef. For those people, especially if you can give a price point under that of beef, they will very happily use it. And I think that category will definitely eat into the meat sales if there is a product they can accept.