This could be true, however I think that if all this torrent streaming sticks to same behavior such as that of XBMC torrent [1] (i.e. streaming for the whole duration of the movie) the swarm will end up stronger, not weaker.
Suppose there is only one seeder and 2-3 leechers. All leechers managed to get 50+%. Now, if the seeder disappears, the leechers should be able to finish the download from each other. If all initial leechers are streamers, that download will never finish for them or anyone else.
Supposedly long seed intervals will not help the swarm in that case.
In reality, there are 100+ seeders and if you had a constant supply of streamers who spend half their time seeding (assuming they download the whole movie by the 50% mark), then you have a very very healthy swarm.
Supposedly long seed intervals will not help the swarm in that case.