Humans, like our primate relatives, do eat non-vegetable matter. Moreover, there is a case to be made that it is through our discovery of FIRE that our success as a species really started to take off, because while other animals consume their nutrition raw, the act of cooking one's food to break down plant and animal matter enabled the human digestive system to be simpler and less energy-consuming. We essentially offloaded a good part of the function of digestion into cooking, and this is what allowed us to evolve an increased intellectual and physical activity.
That said, the reason for meat substitutes has little to do with theories as to whether we as a species were/are "intended" or adapted to eat meat. It has a lot more to do with the environmental efficiency and cost of producing large volumes of meat for a very large population of humans; moreover, a greater proportion of the human population is increasing consumption of meat due to the general rising standard of living in developing nations.
Therefore, any discussion of the palatability of artificially synthesized meat (i.e., any meat not systematically produced from the raising and slaughter of live animals), must include a discussion of the efficiency of the process. How much water does it use? How must does it cost to implement at large scales? How much electricity does it use? How much organic/raw material does it need? How much does it pollute, and what are its polluting byproducts? And how good is its nutritional profile? All of these questions should be evaluated in themselves as well as in comparison to existing meat production processes.
Meat consumption will never go away unless the supply disappears completely. What matters is not why we eat it or whether we should stop; what matters is how we can do it in a way that is sustainable from an economic, environmental, and public health perspective.