Oh.... Well, I guess I can understand that.... A little. It would be nice for them to make an effort in making a new foal, like they did for the cake twins, but sometimes you just got to accept certain things. This is probably one of the things I would just accept. Yeah, its not the best. But you know what? Who cares? The fact that their probably having a foal at one point in the series is the point. It'll be especially funny to see how Twilight reacts to it. You know how she is.
Besides, its a show for little girls, and they probably think the little ones won't really notice, but will be more focused on the fact that they had a baby.
It's business logic, best not to question it, or you end up fussing over something that, honestly, probably doesn't really matter in the long run. Besides, her style may change between the time of that post, and when/if they decide to pull it off. Maybe Hasbro will decide to have them have a colt instead. Or maybe they'll decide Cadence should have twins. Who can say? It'll be interesting to see unfold, if nothing else.