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..
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.