Simply seeding for the 1+ hour you are watching the movie is typically enough to keep a torrent swarm healthy now that the majority of users have 10 Mbps+upload speeds. In that time, you can easily upload more data than you downloaded, meaning you are a net benefit for the swarm.

In a general sense, any video content where the bitrate is lower than the typical users upload bitrate could work for a webtorrent only swarm of 'regular' users who aren't going to do anything special for network health.

Problem is: they can't watch movies in their browser because they can only leech from people who are seeding from a webtorrent client.

Let's say I have a movie I want to let other people stream with their browser. I am obligated to use some javascript horror webtorrent-compatible client. If I already have a seedbox running transmission I simply cannot use that for this purpose, which is a shame