I think it's pretty clear that optical viewfinders at some point will be replaced by electronic ones, meaning yes, SLRs are doomed in the long run. That doesn't mean they will become entirely extinct (like there are still LPs around today).
Regarding camera size... there are certain limits every future camera will need to adhere to, that can never change, no matter how clever future engineers are. I'm of course referring to the laws of physics. It may sound scary, but is often really simple, like this one: A sensor can never detect more light than is fed to it.
One implication is that we cannot expect sensors to increase their sensitivity indefinitely. Sensors already have a quantum efficiency (QE) of about 50%, meaning about half of the photons of the right wavelength hitting the sensor will get registred. For Bayer-type sensors we can thus expect at most a factor 2 improvement in sensitivity, even for an ideal sensor. With photo-sites that can detect all colours (as opposed to one colour; think foveon-like sensors), we can gain another factor 3 in sensitivity.
Low-light photography dominated by thermal/readout noise can naturally benefit from eliminating those additional noise sources, but the photon noise will always be there no matter what.
Another implication is that a larger lens will always have a low-light advantage, no matter the size of the sensor. Thus you have the option to choose a big lens that will be good in low light (and give shallow DOF), or a small lens that will fit easily into your pocket, but deliver deep DOF and not work as well in low light conditions. This will always be true. Forget about small lenses delivering shallow DOF - the reason the DOF becomes shallow is that the lens is big! Simple.
For perfect optics, the size of the sensor does not matter. In reality, it is much easier (and cheaper) to design satisfactory optics for a large sensor than a small. That's why the EF-S 17-55/2.8 is slightly more expensive than the EF 24-105/4.0L, eventhough the latter sports better build quality, has a larger effective aperture (26 mm at f=105mm compared to 20mm at f=55mm for the EF-S), and has the red ring.
Since we can expect sensors to continue become cheaper I would predict that sensors in general will become larger, and that the optics will become cheaper. I'd be surprised if we hadn't cheap FF P&S with plastic lenses within 10 years, producing better quality images than current P&S to better ergonomics and much better prices.
