I think what prevents this from being "parody" is that the thing being copied is not the thing being parodied. A "Downfall" parody video about Star Trek isn't a

I think what prevents this from being "parody" is that the thing being copied is not the thing being parodied. A "Downfall" parody video about Star Trek isn't a parody of Downfall, it's a parody of Star Trek, or maybe a parody of Star Trek fans. Similarly, when Weird Al Yankovic ripped off Nirvana to make fun of Nirvana he didn't need to get permission to do so, but when he ripped off American Pie to make fun of Star Wars Episode 1 he did. (For the record, Weird Al nowadays always gets permission from the original artist anyway.)