How would this work well? I kinda thought that the whole benefit of torrents was being able to parallelize the downloads in chunks across the entirety of the file - wouldn't this negate that?

(Ab)using torrents for sequential watching is not an original idea, probably the thought is "doesn't matter, I get to watch the movie".

I think the original concept of torrents is to ensure the chunks with the least amount of copies in the network get duplicated earliest (in worst case there's only 1 copy, if that peer goes offline you won't get a complete download).

But this concept would be interesting to see when people stop watching.. if 1000 clients have the first 20% of the movie but only 600 have more, it means many quit watching at 20% of the movie..

Parallelize in chunks, yes. Across the entirety of the file, not as much. If you work in parallel across a 50MB range it works about as well.