The main point, that comparisons with Nintendo's past systems and Sega don't really apply here, has already been made. I certainly believe there will be a PS4 but not because "Nintendo still made Wii after GC" etc, that's a totally different situation for a whole host of reasons.
Also I think some people need to be more careful to distinguish between profiting at a given point in time versus profiting over a longer period of time. The PS3 is, and by all accounts has been, profiting per systems sold for several months now, that's not a debate. PS3 is CURRENTLY profiting but whether it will be profitable across it's lifespan is a totally different kettle of fish. The PS3 has dug itself a 5+ billion dollar hole (not sure of the exact figure) which makes the chances of it ever breaking, even if the 10 year plan comes to fruition, pretty unlikely given that the PS2 in it's best year barely made a billion.
I think a big thing with revenue for nintendo. Is that they make the software that sells some of the most out of any software company. such as New Super Mario Mario Bros, Pokemon. And several others that are around the 10 million mark. Not just licensing games.Sony's big games (I'd say are comparable in quality but to a different crowd). God Of War, Little Big Planet, Uncharted for example sell in the only 3-5 million range. While nintendo has lots of games, hell some of nintendo's lower quality games probably sell about that much.
Also, the hardware for PS3 was outrageously expensive, in costs for sony, although it basically secured the blu-ray market (now I know sony doesn't own all of th blu-ray IP or how exactly royalties work but I assume theres some benefit for doing so) which should also be calcuted into whether the ps3 made money, because without the ps3 there's no saying that HD DVD wouldn't have been the next disc format. (Yes digital downloads, could be the future, but I can't afford a connection that downloads HD movies reasonbly, but it could be possible in the future).
One last thing, is Sony, and Microsoft don't have all their eggs in basket of videogames like nintendo. Beings everyone knows what microsofts main business is. And I believe (not sure) the videogame part of sony is around 20%. It's the company's decision, of whether to come back or not. And they still have a significant market share, that if they do the next gen correctly and don't force losses on themselves on every console for the first 3 years. They'll be in profit land I think. :D