In addition to the frames having to be all new, and they would all have to be new, as stretching the upper deck would affect the ones in the present hump too, as the present upper deck tapers off from the front to the back, the significant increase in weight would necessitate an all new wing, which is another big ticket item. They would end up with the cost of making a new airplane. The upper deck is quite small. It is only 6 seats abreast in economy, and even that is cramped. They could get the same extra seat capacity by stretching it, and that route would be more feasible.
An optimist robs himself of the joy of being pleasantly surprised