You totally CAN disable all uploads in the torrent protocol. Just set the "upload budget" to zero in most clients. Just nobody realizes they can do that.
Bittorrent is wildly successful in part because every popular client makes it nontrivial to "opt out" of it's more socialist components (chunk trading, DHT participation, seeding by default).
Making an "leech behavior only" torrent client is straightforward and viable.
