My guess is that they needed a figure to fit the DRG character?

What if Estinien isn't the DRG then? Who else would take up the mantle? And if that person comes up, you would also be puzzled because YOU are the Azure Dragoon now and no one else. So why is there another DRG coming out of nowhere AND mentioning the Eye?

I think Estinien is there to symbolize the DRG's involvement in defending Ishgard, as their duty is. It would be weird if they don't mention a DRG or conjure one up in the storyline.