If all armrests were placed in the upright position when passengers were boarding, everyone would have a much easier time getting to a window or middle seat and boarding (especially for B and C groups) would be much more rapid (and easier). Ditto for exiting.
The current problem is that aisle armrests are a bit difficult to raise up. However, many (most?) SW passengers are pretty savy, and if shown once how to disengage the little lever underneath the aisle armrest, they will probably remember how to do it the next time they fly and are in an aisle seat.
Alternatively, cabin staff could raise all of the aisle armrests as part of their pre-boarding "clean up" activities.
In either case, the directive to lower all armrests could be added to the usual pre-departure direction to "make sure your seatbelt is buckled," -- as in "make sure your seatbelt is buckled and your armrests are in the down position."
I don't see that as being very much different from the pre-landing instructions to "raise your seat and return your tray table to the upright position."
I often sit in an aisle seat and keep my aisle armrest up during the flight to allow me to exit my seat easily when I need to stretch or use the restroom. The raised armrest also allows my fellow passengers in the middle and window seats to exit the row easily when they need to use the restroom.
