The problem is not the protocol but the lack of features like subtitles. And torrent contents not being standard. All this can be easily scripted to cover most cases.
Unlike simple HTTP streaming, the clients can use this spare bandwidth to download some blocks from the end of the file, even when streaming from near the start. So a sensible torrent streamer can still ensure that later blocks are not too rare.
If you download from front and back simultaneously only the middle blocks would be scarce and you still get at least half your original streaming rate. And no weirdo will stop watching midway through a movie.
That being said, though, do streaming clients make much of a contribution to the amount of seeded data?