The claim 'Pluto is currently inhabited by five hundred and thirty-eight witches' is at this moment unfalsifiable. Does that mean that denying such a claim would be "all handwaving and speculation"? If science can't make predictions about incompletely known phenomena, but can only describe past experiments and suggest (idle) future ones, then science is a remarkably useless thing. See for starters:

• Science Doesn't Trust Your Rationality
• Science Isn't Strict Enough

Sometimes a successful test of your hypothesis looks like the annihilation of life on Earth. So it is useful to be able to reason rigorously and productively about things we can't (or shouldn't) immediately test.