Rather than a termination of interoperability, it's an extension thereof – whereas before, to chat with AIM contacts from Gchat, one needed to log into an AIM account – effectively making Gchat a "google-themed AIM client", at least while talking to AIM users.
Going forward, Gchat users will be able to see, chat, and possibly use other features (file transfer, video/audio, conferencing, chat rooms) with AIM users without needing a separate AIM account. That is, so long as you use Gmail or Google Apps, you'll be able to delete your AIM account from your Google Chat preferences(*), and still chat with AIM users.
Presumably, Google won't remove the existing functionality outright for a while, so this primarily affects Gmail/Gchat users who don't already have an AIM account, who also need to chat with someone on AIM. That sounds like a pretty small group of people to me…
Thinking it through a bit further, Gchat is (last I checked) based on XMPP protocols – and for this kind of AIM integration to work, it's possible that AOL has rolled out (at least in a limited fashion) an XMPP gateway for AIM, or even have transitioned to XMPP on their backend. If the latter is true, then this announcement is just the first of many, and ultimately will allow for many more uses for an AIM account – and perhaps, continued relevance for AOL, for another few years, at least.
(* Aspoints out in the comments, it is not possible to delete an AIM account/screen name, well, ever. Yikes!)