I have to be honest, I have never sat in downstairs J, only Y downstairs and J upstairs. Your points are interesting, and also the previous one about travelling alone and being in the F seat to avoid the middle passenger going past you to get to the aisle, as they are likely to be travelling with the person sitting in D. This all makes sense. My son is booked on QF10 SIN MEL next year. I would think this flight would normally be pretty full, but the "downstairs" point of view is starting to make sense.

^