You’re correct about seeds, but peers who are also downloading will often stop sharing with you if you stop sharing with them. Seeds generally are configured to try to give different pieces to different peers so that they can send them to each other and reduce load on the seed; they don’t want to give you the entire file directly unless you’re the only person downloading. And peers prioritize and filter which other ones they’ll send pieces to based on reciprocity.
replyYou will probably get the data eventually, and it really depends on the composition and configuration of the swarm, but generally, you do need to upload if you want to ensure the fastest and most reliable download.
Long-running torrents are mostly populated by seeders. Bit torrent was originally designed for a lot of downloaders to get a file at the same time with limited seeding bandwidth, so leechers would need to trade with each other a lot, but that's not really the situation most torrents are in today.
replyIt depends what you’re downloading. Peer prioritization mechanics have been relevant for a lot of my recent downloads.
replyYou can, but you will slow down your own downloads dramatically by doing so. In some cases you will fail to finish them.
replyThe case for doing this would be just so you can have this ridiculous legal defence Meta seem to be trying to pull out. Really no other good reason. Even for the most parasitic leeches, zero upload is a bad strategy.