Cathay Pacific did this years ago with economy clamshell seats, i.e slide forward, but apparently aggregate consumer feedback was bad and they were all withdrawn. Shame.
Essentially the seat did slide forward and the back moved down with it, but the problem was that the belt was fixed in place in the seat shell. For me that had the effect of driving the belt into my kidneys if I moved the seat more than a few degrees off fully upright.
More likely the sliding seats just weren't very good.