Originally posted by Aristophanes It's not a must. It will simply be as cheap to produce and implement as APS-C.
Why then, would one knowingly use an inferior size?
I doubt that FF will ever be quite as cheap to produce as APS-C. (And if so, why wouldn't MF be as cheap to produce as FF?) Nor does the whole equation reduce itself to the issue of superior size versue inferior size. There are tradeoffs between FF and APS-C; it is simply not the case that FF is absolutely superior, even if costs are ignored. The 1.5 crop factor, for instance, can be quite useful in reducing costs to those needing longer focal lengths. On an APS-C sensor, a 300 f4 lens has the FOV of a 460mm lens. To get something comparable on FF, you'd have to a get a 500mm f4 lens. Such lenses are 4 to 5 times more expensive. That's a rather important advantage for the hobbyist who merely wishes to photograph the miscellaneous critter.
Not only do FF lenses tend to be more expensive (sometimes greatly more expensive) than their APS-C equivalents, they tend to be bigger and heavier.
The majority of DSLR consumers are hobbyists. Many of them cannot afford to spend a lot of money on a camera system. For them, an FF system does not make financial sense. The "sweet spot" for them is what they can afford. And that means APS-C. In the final analysis, it is economics that is going to decide this issue, not the technical superiority of FF.
Also: this whole notion of using APS-C lenses on FF bodies is a non-starter. If camera companies suddenly abandoned APS-C, many people who have invested in APS-C lenses would deeply resent it. Nor would the ability to use such lenses on FF bodies provide much in the way of consolation.
There's no reason why an advance, industrial global economy cannot support both APS-C and FF. Why should one triumph over and obliterate the other when both fulfill the needs of different camera enthusiasts?