Will Steam Machines or PS4/Xbox One take 4K into the mainstream?

It's going to take a fair bit of optimization. The PS4 and Xbox One can barely play current titles at 720p upscaled to 1080p, much less 4k.

Missing from the article was a discussion that the SD-HD conversion was driven by a different form factor... the mere presence of a flat panel widescreen as a way to show off someone's willingness to spend money.

4K isn't going to have that major (primary?) draw because it'll look exactly like a HD screen when its shut off.

Aside from the whole "not improving the gameplay or the experience" thing.