When presenting an idea to a professor: Be optimistic or realistic
Be realistic. A seasoned professor has (by necessity) developed a keen BS detector (required to wade through teaching, at very least). A (prospective) student who doesn't know how far they can realistically go will either turn out reckless (and dangerous to the project) or get frustrated when things don't work out, perhaps dropping all (and thus being dangerous to the project). As the professor in turn stands of falls with his successes (and "my PhD student failed me" is no excuse, really), it would make you a risk not worth taking.

Yes, it is nice to show that you know where success in the first step(s) could take you, but they might fail, or turn out unexpected results leading elsewhere.