It certainly wouldn't be fun creating many models for overworld usage, but once that hurdle is overcome it likely wouldn't be so difficult to implement the follow me feature in future games. Plus, they likely wouldn't need to create models for the larger Pokemon, since the sizes are more accurate than in past games. That being said, I didn't expect GF to include it in XY. They already had to make a good number of models (I heard some models came from or were designed after the models in Colo/XD; dunno how true that is, though), so having to make ones for the overworld likely would've taken a lot more time, thus resulting in a much later release date for XY (and ORAS) ;;