It is true that they need to port/recode it. They also need separate server farms, and they need to handle sales, marketing and distribution somewhat separately. Those are costs. They are paid for by the additional sales you get for having an old gen version. When those sales fall to the point that the revenue does not pay for the costs, it is time to stop supporting old gen. But this is unrelated to "holding the game back." The main difference is graphics. You can have the same size areas on old gen. You can have the same fights and the same weapons. You just can't make explosions, smoke effects and the like look as good or the game run as smoothly when a lot is happening. That is accounted for in porting/recoding. Basically: this discussion is about a cost-cutting move disguised as an "advancement."