A vegan diet can't give you enough calories to survive
Most of the world's calories already come from plants. Rice, wheat and maize alone supply roughly half of them. Plant foods like nuts, oils, grains and pulses are as calorie-dense as meat or more so. The real risk is under-eating when you first switch, and that is fixed by eating more of the right foods.
Meat is a compact, reliable source of energy, and when you remove it you remove a big part of what many people actually eat each day. Plant foods are bulkier and more filling, so it is genuinely easier to fall short on calories without noticing. For anyone active, growing or already lean, that shortfall could do real harm. Saying you'll get your vitamins from a pill doesn't answer whether you'll get enough food.
Calories are not a special property of meat. They come from carbohydrate, fat and protein, and plants supply all three. Globally, rice, wheat and maize alone provide around half of the calories people eat. Many plant foods carry more energy than meat does. Nuts and peanut butter run to around 600 calories per 100 grams. Cooking oils run to nearly 900. A cooked sirloin steak comes in well under half of that. Dried lentils, oats, pasta and bread sit in the same range as most meats or above them.
There is a real point underneath the worry. Whole plant foods tend to be high in fibre and water, so they fill you up on fewer calories. Someone who takes meat off the plate and replaces it with a bit more salad will eat less than they need. That is one of the most common reasons people feel tired or lose weight after switching. It is a planning problem, not a biological limit. The fix is to build meals around energy-dense staples: grains, beans, tofu, nuts, seeds, oils, avocado, dried fruit. Athletes and people trying to gain weight do this routinely on plant-based diets.
The major dietetic associations judge well-planned vegan diets adequate at every life stage, and energy intake is part of that judgement. Large population studies do find vegans are leaner on average. That reflects lower calorie density, not starvation. Malnutrition on any diet is possible if someone restricts severely. When that happens, the restriction is the cause, not the absence of meat.