I don't believe a smaller sensor will ever have an advantage over a larger one. It has always been the case that a good big-un is better than a good little-un. Convenient and cheaper maybe, but for image quality at the end of the day, the bigger the captured image the better. It was always true with film and I'm convinced that it will remain true in this modern digital world.
It's interesting that Canon are said to be interested in acquiring a medium format foothold. While the Japanese have made the 35mm "full frame" size their own, the same is not true of larger formats. Pentax have at least made a start but Canon and Nikon seem to be a bit slow getting into the game.
Once digital technology matures, I can foresee 1.3x and 1.6x crop frame becoming a thing of the past.
Given the economics of semiconductor production, smaller sensors will always have a cost advantage over larger sensors. Since an APS-C sensor is good enough for many uses, we may well see crop frame sensors continue to be used.
Where a crop sensor has a distinct advantage is in areas such as wildlife photography on foot. A photographer can carry lighter and cheaper lenses, and still get a chance at getting a shot. (Imagine lugging a 600mm lens on foot across mountainous terrain with a few days' supplies and a tent in your backpack!)
I think the main driver of crop frame sensors will remain cost. For most of the consumer market, a crop frame sensor is good enough. This is because the resolving quality is good enough to produce the print sizes most consumers want, and many users are not concerned about the benefits of a full-frame sensor, such as shallower depth of field or better low light performance.