Who has tried swimming (slithering) upstream on the seat backs (as opposed to in the aisle).

And why can't those coming forward give you isolated times of just 1.0 second to let you swim upstream in the aisle one row at a time instead of instantly coming forward to fill all available space? They can make up for the lost time very easily because after granting you that second the space further up front will be even more empty. Unless they really specifically want to gloat that you will be last off the plane having booked a seat in the first 5 rows.