I think the main challenge regarding this issue is to provide support for multiple resolutions while keeping the same viewing area for all players. I believe that's why we all have the same fixed aspect ratio. On some other online games, the ability to change aspect ratios might give a slight advantage to wider monitors by giving some extra viewing area on the sides. So maybe CipSoft is still not sure how to provide a better support for wider monitors without giving such advantages as being able to see some extra few sqms to the sides.
I think you're on the right track for a solution. Again, in my opinion, the best approach to a better support to multiple resolutions while keeping the same aspect ratio for everyone would be to remove the side bars and use floating windows for inventory/battle/character info as you showed on your example. However, unless there is a client revamp on the horizon, I can't see this happening very soon.