That is plainly not true. Actually lots of people uses AIM - at least here in Norway. Works nicely with iChat - so no reason for Apple to make a completely new client (AIM is actually a pretty good one also!).
There are also several other clients now collecting all kinds of protocols - and MSN and Yahoo are in talks about interoperating capabilities. Which means these guys are not indifferent to the idea of opening up to Apple as well. IM is going to be big - a lot more important then even e-mail as it also goes mobile now. So to not confuse the users totally they start to open up the protocols to each others. MS relies on its coming integration to office apps - just like iChat actually works with adress book, mail and such.
I really believe that Apple works on getting protocols for at least MSN and Yahoo in there... maybe also Jabber and some of the lesser ones... Apple-users are not any priority with MSN or Yahoo anyway - but hey if they can chat with their windows-clients, why not? I think that is the way - iChat stays Mac only - but gets multi-protocol. I hope