It's much more of a storage space problem and a device running 24/7 problem. With the desktop of yore you can just leave them running and attach cheap HDDs to them for media storage. Nowadays, most people have laptops -- which are explicitly closed for most of the time -- and phones -- which doing 24/7 upload on will kill your battery life. And neither support 3.5" HDDs.

BitTorrent is certainly built around the idea that upload can be slow. Even if user have upload 100x worse than download, 100 seeders can saturate a leacher, and then the leacher becomes a seeder.

The problem is just that he doesn't stay a seeder forever, because of the other problems.