Adding F seats doesn't just benefit award flyers. For example, CX doesn't bump award flyers if a "paying" pax wants an F seat (subject to capacity controls it's first come first served), so added seats would allow CX to accomodate more "paying" pax. And don't forget that CX still gets paid for award tix issued by another carrier (eg, AA, BA).

But, yeah, I don't see them adding seats to existing aircraft. Too expensive.