The problem is "unobtainium" is used for a hypothetical fictional material to solve an engineering problem. In the context of this movie its not hypothetical. it exists. they have samples of it. They would have an actual real name for it.
Its the Magic paradox. Magic wouldn't ever actually be called Magic in world it existed in. It'd be Mana, the source, the force, or some other shit. they would have an actual NAME for this specific energy or system, in the same way we have electricity, or chemistry, or physics. but Magic stops being Magic the second its an actual observable and replicable phenomenon. the second that happens, its a natural science.
While it is certainly true, we don't always name things in the most competent or expected of ways. "unobtainium" being a nickname, that became its official name is certainly possible. we are back to, "that sounds retarded". They should have just used some persons name, and said "named after the person that discovered it. "Jonesium. named after Doctor Randall Jones."