Reference the 500k
1. Many players have two or more accounts. (Note accounts not players) Another little known fact is that players expressed the desire to be able to play several characters at the same time, so there are no restrictions to number of accounts you can have, just a different email per account.
2. The forums have many posts of players returning. Some just for a look to see how the game is progressing or to look up old friends.
3. About 2. It is a lengthy process to restore an account if it is deleted, and frankly time is better spent looking into bugs/fixes.
4. Would you rather this game did the same as many other games;
A. You must log in at least once a week or account is deleted.
B. You must play within a year or account is deleted.
We would rather keep the accounts so returning people can pick up where they left off, or if they so desire make new characters, this game is as has been said many times, a game in the making, yes it is taking time, Devs move on as RL dictates their availability, so new have to be found, who in their free time can help with making the game.
So it is a low priority to remove unused accounts, but nothing stopping you asking for your own to be removed, and when time permits it will be.