Someone added a tag for mcguffinite that states it to be "the reason". The linked page invented the term to refer to the substance being sought after, with the reason implied as coveting or requiring that stuff somehow.
So Interstellar is not an example, as other kinds of reasons are not covered. I really don't like the application of the term to this larger idea (the reason) because it does not fit the grammar or etymology at all. McGuffinite would be a natural resource substance, like unobtainium. It's not a social urge or drive or a trope, like "the Earth is dying".