Aside from the specific issues about discomfort due to poor design detail, a fixed back doesn't actually make much geometric sense in the cramped conditions of Y - although fine for J or F.

Just think about the situation when all the pax have their seats reclined. Then they're in the same position as with a normal seat, except that there is a triangle of space between their back and the fixed seat back which would just be empty space with a normal seat - which would mean more room. In other words, you get less legroom with the new seat (or really, as others have said, less knee room) when reclined for the same seat pitch.