Considering that cd-keys weren't tied to accounts and botting and item stores were a huge thing, just imagine how many accounts there were when people like me had 6 or 7 at a time.
There were estimated 17million copies of D2 and 17.5million copies of D2:LOD sold. I got some basic data for the other guy who thought that all the accounts would fit on an 80GB harddrive. If they can cut down the time that stuff takes by deleting millions of accounts that haven't been logged into for over 10 years. Who knows what the current process is, but I imagine they're still paying a few people do it. for each of the major realms(I haven't been on in a long time, but I assume there's still a US EAST, USWEST, etc.). Someone has to manage and monitor backups and run restores when a drive inevitably dies. They're a company who have to protect user data if they have it.