People are explaining.
Look this is just straight forward financial planning within a business. FF14 makes a certain amount of revenue from core game sales and subscriptions. It has certain base expenses to keep the server infrastructure and support services running. What is left over after that is used to determine the games performance financially and allocate a budget to the developers, taking into account things like necessary upgrades and alterations needed for sustained subscriptions and growth. That budget is then divided up into the various development projects used to produce in game events, expansions and the patch cycle and work on infrastructure upgrades and game features.
To expand their revenue they created the cash shop. Rather than determining the net revenue by factoring in infrastructure costs, net revenue is determined by the production costs. SE forecasts what they expect sales figures to be and determine if the production costs are sufficient to justify the production of the items for sale.
Now here is the interesting bit often overlooked. Net revenue from the cash shop would be factored into the total earnings of FF14 which in turn will dictate the core budget for the main content production and will help justify investment into the game such as server upgrades. The cash shop helps pay for stuff you get for your sub fee, not the other way round. Additional revenue streams help the developers put more into the game, not less.
That is a very simplified version but almost certainly close to what SE's model is from the information I have seen.
Sales forecasting is a basic part of developing new products for sale. They would be using a lot of existing metrics to determine what is in demand and likely to provide good returns. This goes for the base game as well. The features they have developed and the focus of content and design will have taken into account what players are likely to react positively to and enjoy.