I know this problem
Still they would loose a lot of potential resubscribers if they bluntly delete accounts (especially at a regular intervall). Just by playing or reading here, you will notice many returners when something big changes (khitai, new game director, turan, 5 years etc.). Those came mainly from the period you want to delete.
But what they can do, is notify the owners of the old accounts first, set a timer and if they do not resub or log in or the adress is not valid...it gets deleted (or the chars names changed). If the timer is long enough, no one could complain especially if such a measure is announced on the website and forums as well.
About the names, they can do even more (without such drastic measures). For example change the names adding the typical number codes...
I am sure, they will come up with something like that when they either merge all servers or do "one server" tech (they will need to do a namecheck routine anyway to avoid trouble and could implement such a name addition/check). And...maybe they already did that?