Many torrents are on multiple trackers and, via the almost ubiquitously supported announce-list extension, have multiple trackers listed in the torrent file its

Many torrents are on multiple trackers and, via the almost ubiquitously supported announce-list extension, have multiple trackers listed in the torrent file itself. So those 3M clients are unlikely to have been sat doing nothing because this tracker had vanished, they would be participating in the swarms around the other trackers listed in the metadata for each torrent.