I can't predict what Apple might do, but here's something to consider: The dual-lens camera assembly is larger and more costly than the single-lens assembly. Therefore, space must be found, and the added cost must be offset by other cost reductions in order to keep the price in line. If those cost reductions can be found, the question is whether there's another added-cost feature(s) that more customers would find attractive than this particular camera.
The single largest user of space is the battery, and it is the component that is easiest to make smaller. Making the main logic board smaller would likely require a reduction in the component count (one chip capable of substituting for multiple chips, etc.). If the chip-makers do not produce such a product, perhaps Apple could design one of their own. Otherwise, the answer might be a reduced battery capacity, which is not likely to please the masses.
The primary reasons for preferring the smaller iPhone to the larger are lower size/weight, and lower price. If one's reason for wanting this camera in the standard-size iPhone is size/weight, I get it. The Plus is too large for my taste. If one's reason for wanting this camera in the standard-size iPhone is cost... Everyone likes to get more features for less money, but from a manufacturer's standpoint, they'd need to come up with a new incentive for people to spend the extra money on the Plus. Perhaps that incentive would be an even more attractive camera/lens.
