I guess people who buy protein powders are idiots? The whole point of "a well balanced diet" is to eat a variety of things to get all your nutrients. This product is simply another means of getting protein in your diet.
The comparison might still not be perfect, but OPs point about needing a well-balanced diet is still relevant and important to this discussion. We don't get all our nutrients from a single source, especially not meat. So not getting all our nutrients from lab-grown meat isn't a problem. We'll get them from somewhere else. Unless you know there is a specific nutrient in meat, which we don't get from any other source, which lab grown meat won't replicate. However, that's unlikely or vegans would all be dead.
So really, what's the problem with replacing meat?
Also this product will sooner or later be marketed as legit meat replacement.
But otherwise yes, even those who'd eat it just for protein would be idiots because eg whey comes from a natural food source beforehand which means nature has already done it's thing properly beforehand.
To think that growing stuff in a lab has no unintented consequences is beyond stupid, even if you use it "just for protein".