Bittorrent and other decentralized systems are not good choices for unpopular data. As another commenter pointed out unpopular torrents will only be seeded by archive.org.
I'd argue that bittorrent is still better for unpopular data then direct downloads.
I think BT makes downloading and sharing accessible enough that more people would be encouraged to download more than they normally would and it is for sure easier for someone to see a request for a reseed for an unpopular torrent and load it up onto their tracker for a few hours than it is to try and rehost it.
Surely if one other person is downloading the same torrent at the same time it's better (potentially) than a direct download, as part of the torrent can be shared in the swarm..
As long as archive.org is seeding everything all the bits are there..
Something like Freenet would be better for this I think, since the "what" you share is limited by how much space you give the datastore, unlike bittorrent where you consciously decide what and what not to keep seeding.
It still has a problem with non-popular content being hard to find, but not nearly as much.
