There's a simple reason for keeping an AOL/AIM screen name. For instance you have an elderly friend or parent you need to stay in touch with who will never update to something of today standards.
Or you have your own name or business which otherwise would not be possible on any other email service. Why get rid of something that works reliably for certain purposes?
Now my next question is, will I lose my MSN sign in to iTUNES and that Apple ID next? I'm already having trouble with it not working in other new Apple services like Messenger & FaceTime.
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No, this means you're a young punk with limited knowledge.
I chat with elderly people every day on AOL/AIM. These are people that will never change. Don't look down upon people who didn't live your young life. Be thankful you know more.
I wasn't looking down on people using AOL (did I even say anything about them?), I was rocking AOL back in the day with a 2400 baud modem, and connecting via multiple phone lines (merged connection) up into the early 2000's. If people have a use for the services AOL offers, then great, but AOL itself has largely stagnated and doesn't bring much to offer the Apple ecosystem anymore I'm surprised Apple has keep the ability to use an AOL account for this long. As far as I know AIM support will still be in Messages/iChat just not as an extra login to things like iTunes.