Just because vegetables, lentils or nuts contain protein it doesn't mean it's the same/equivalent to the protein in an animal product.
Meat is actually super easy for humans to digest and it has no downsides to it. All vegetables on the other side contain plenty of anti-nutrients such as folate and oxalates.
Everything in human body, skin, connective tissues, tendons, hair, nails, muscles is essentially built out of protein and collagen. Fats are essential for hormone function.
I guess I'd challenge the 'no downsides' claim. Few people stick to super lean grass-fed cuts, and the picture on the site is even a ribeye steak :P
The protein density (g/kcal) of a ribeye steak is basically the same as tofu (I think like 14g/100kcal vs 11g/100kcal in tofu)
I know I'm moving the goal posts slightly (I admit I didn't know about bio availability, and see now that I have more to read up on e.g. Broccoli), but am learning as I discuss rather than arguing a fixed point.