Whether or not the money generated from the mogstation goes directly back into XIV is entirely irrelevant. If XIV is generating more income, SE is going to be more willing to increase it's funding, bottom line. It doesn't get any more simple than that. If something is doing great financially, you're going to invest further to multiply those earnings. Just look at Stormblood - the budget was tripled over Heavensward.

Cash shop purchases directly influence that, as they net a lot of profit for SE. By purchasing from the mogstation, you are directly influencing the game moving forward in a positive way. Keep that in mind.

And of course, as has been mentioned countless times before, you don't have to buy anything from the cash shop. They already add a ton of content to the game in the monthly updates, one or two emotes every half year or so is a drop in the bucket compared to the actual content you get for paying your sub.