...not deleting characters is the absolute most guaranteed way to ensure that players that left are willing to come back in the future. I don't know that any of the major mmorpgs have ever deleted a character.
FFXI. ;P After three months. After three months they just delete your account
-- if you wished to play again, you'd have to purchase a brand new client! Which is probably one of the reasons that I didn't play. It's *certainly* the reason that I couldn't locate a used account.... :?
I can go back and see my Anarchy Online characters even today. It's been at least a year and a half since I've played, if not longer.