It won't really matter that much because you'll be going through all the areas for the story, since end game will most likely again be tied to completion of said story. As well people can run fates in Coer right now and still get to 50, yet people still do so in Northern Thanalan as well.

You also forget that the area, even after it loses its shiny, will still be newer and thus players will probably like being there more often UNLESS it just ends up being horrendsly bad for leveling.