This will happen but not any time soon .
The started up cost alone would be crazy high , the internet infrastructure in not ready in ton of places .
If a DD only console can't make right now in today market must less streaming only .
We're not talking about streaming-only consoles.
Instead we're debating what will be available on that service and where it will be available.
With regard to the 'what' - PS4 is the only software platform that they'll have the from-scratch opportunity to license for cloud delivery. It'd be easier to make happen on a technical level than PS3 content. I've little doubt they'll make PS4 content available on the service - whether it'll be the whole platform or not, I don't know, but I wouldn't be surprised if it'll be more or less a universal first-class delivery mechanism alongside retail disc and digital download. (Citation in support of the idea of contemporary/PS4 games: "SCE will deliver a world-class cloud-streaming service that allows users to instantly enjoy a broad array of content ranging from
immersive core games with rich graphics ", "As a new member of the Sony Computer Entertainment family, we're working even harder to ensure the
world's best entertainment content is delivered instantly to you")
As for the 'where' - gaikai's job listings might give a hint. There seems to be the suggestion that the main client will be a webkit or html5 based piece of software that will accomodate wide range of devices. "As a part of Sony Computer Entertainment, Gaikai is leading the cloud gaming revolution, putting console-quality video games on any device, from TVs to consoles to mobile devices and beyond." with work 'Integrating with other technology stacks and application components from other Gaikai teams, other SCE teams,
and various device manufacturers and middleware providers." The Sony acquisition press release talks about the delivery of Sony's cloud service 'on a variety of internet-connected devices.'
Now, maybe it'll just be Sony internet-connected devices - Sony TVs and Sony consoles and Sony mobile devices...perhaps 'various device manufacturers' is a reference to legacy contracts Sony still needs to fulfil post acquisition, but the simplest and most flexible approach for Sony would be a web-based service/client, and if that's the case, I don't see them holding it to any one device or set of devices for very long, if at all.