I am facing a similar issue for near future. I really think we do NOT need a window there. So it WILL get removed. Your issue is: when? I would do it now when you redo the entire bathroom. It would not be heard to install some exterior covering now that can be covered again later whenever you do the exterior siding. If you delay, though, you WILL have to figure out how to fill in that window hole inside to match the recently-redone rest of the bathroom. That would be difficult.