Only about 25% of your daily calorie intake should be from protein. 60g a day for a 160lb sedentary person. Those high-protein shakes are usually around 60% protein. It is possible to consume too much protein.

So I say that it's a supplement rather than a replacement because it's not replacing a balanced meal. It's supplementing protein. A single protein shake can give a sedentary person almost 100% of their protein. That's great for active, athletic people who have higher protein requirements. Not great for sedentary people who have lower protein requirements.