I can't see it as a step backwards when it provides great functionality to hundreds of millions of people. I doubt iMessage would be anyone's sole IM platform, and in fact the OSX client supports AIM XMPP as well, so it's really more of an enhancement (where available) for iOS users. Apple isn't really trying to solve problems for people who don't buy their products. At some point in the future I could see AIM and XMPP being added to the iOS client to be used as a fall-back in the same way SMS is used now. So eventually there may be one client that is basically a proxy to various other services and just sort of figures it all out for you.