There's a big difference between "this project is like time travel or cold fusion; it's doubtful whether the laws of physics even permit it" and "this project is like heavier-than-air flight; we know birds do it somehow, but there's no way our crude metal machines will ever match them". I'm confident which of those problems will get solved given, say, a hundred years or so, once people roll up their sleeves and get working on it.