The idea of a lettuce tomato onion burger stack on a bun is that there's a taste and texture contrast between the stack of vegetables and something that's not another stack of vegetables but mashed. (Doesn't mean it has to be Beyond or Impossible, see last para.)
I suspect displacement happened because folks paying $25 for a "burger" when out are less interested in the garden mash, want something a little edgier. FWIW, before Beyond and Impossible, at any restaurant or diner where I took time to explain the difference in alt-burger types, they were willing to try ones I mention below, and they reported customers preferred it. In many, someone had just told them "have a vegetarian burger" and they just got the mashed veggies patty or mashed beans cake since that's what most products are. Not being vegetarian themselves, they weren't aware of other availability. By contrast, Impossible and Beyond essentially went door to door "educating" them, so they switched.
While Impossible and Beyond displaced the meat alternatives, that doesn't mean you have to go meat in a vat at home, you can do products such as Morningstar Farms Grillers Original, or maybe Boca All-American Flame-Grilled Burger. Those taste stack more reasonably with what goes on a burger than the mashed vegetables patties, without the weirdness (fake blood look, why?) of Impossible or Beyond. Your local diner will probably be happy to stock them as they're simple frozen goods.