Wireless is an insanely profitable medium for these companies. "Afford" is a pathetically relative term that actually means "if there is no competitive incentive to do so", then they will stop offering unlimited.
Once the Nextel losses finally stop and Sprint's financials stabilize, you can kiss goodbye to unlimited. I'm just hoping their experimental LTE service will come over here before that happens so I can get in on an unlimited plan and grandfather it as the years go by.