Like K0HTA mentioned, it's up to the staff to decide if it will be a lot of work for no reason, we're here to give them ideas, it might not be useful for you but it's useful for people like me for example. I'm currently part of the admnistration of my clan, it's good to know someone before you add them to your clan, knowing their old nicknames helps a lot in this case.
But I don't think you're going to change your mind anyway, you just can't put yourself at someone's else position apparently.
"besides do you think admins have a long list of the past and present names of people cause if they dont then it will be down to contacting each user. even if this was implemented this way it would be better off to let it slide in and start recording the names afterwards.
it just seems like too much work for a small little feature"
About this, I highly doubt there isn't a place where they can search for old usernames, developers usually keep track of everything going on in their websites or whatever, database stuff and etc.
And I'm pretty sure they wouldn't have to change it individually, actually I'm more than sure they wouldn't have to do this, they could write a code to display the old nickname in that little window and it would apply to everyone, but I'm not a specialist so I can't really say anything about this...
Once again, I don't see a reason to why not implement this, being a "lot of work" doesn't seem like a good reason to leave it undone, it's a small detail but it could help a lot of people, stop thinking if it's useful for YOU, think if it's useful for the others too.