Except people who can afford both fast internet to stream those games, and who can afford the cost charged by OnLive for a subscription, and for rentals and "full" purchases (which is more of an unlimited rental than an actual purchase), can by extension usually afford a decent gaming PC or at least a console. As it stands, you're paying almost as much or more in the long-haul, for a flatly inferior experience because of issues like input lag.
They already sell consoles well below material cost just on the assumption that the person will make them money through the purchase of future games, peripherals, and subscriptions. Furthermore, anyone that can afford a god damn Mac of all things, can probably afford a console, and if they can't... why are they paying for super-fast internet, an OnLive subscription, and a Mac? The only argument I can see in the case of OnLive is like Doomguy said: streaming to your tablet or other mobile device. But even then, a company would need to muscle out other services like Nvidia Gamestream and PS Vita remote play.
Which is really a far smarter system anyway. I'd personally much rather stream a game I actually own elsewhere, through a service like Nvidia Gamestream, than I would "buy" games from a straight 100% cloud service like OnLive only for them to disappear the second the company goes under.