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.

You 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.