Although Nintendo did always support their handheld, even when they had a monopoly, because you're not just fighting for the handheld space: You're fighting for the consumer's attention. Same thing these consoles are doing. A tablet/smartphone is not a console, but people are still spending their free time playing them over a console. Still though, half of what they did on the GBA were SNES ports and their GBC support wasn't stellar either. Hell, I'd argue that Nintendo's 3DS support (announced or otherwise) is even stronger than their DS support, and they actually HAD competition in the handheld space then.
edit: Also looking at how fast mobile technology increases in power, laughing off Android/Mobile based consoles is pretty shortsighted. Amazon and Apple are not the Ouya company.
Then there's also Valve, and who knows what the Steam Machine will do.
Point is, there's plenty things Sony would need to compete against if Microsoft leaves the console space. If Sony tries to pull off that $600 PS3 bullshit again, consumers will find an alternative (ie every other electronic device that is begging for their attention.)