There is a reason it has not been replaced. That is because no other rifle offers a SUBSTANTIAL IMPROVEMENT IN RELIABILITY AND
LETHALITY over the current m4/m16
Why don't they design one. Like they did in the good ol' days.
I remember asking on the Kalashnikov memorial thread if we will ever see another Kalashnikov or Browning in our lifetimes. I don't see why not and I don't see why cost is an issue when it wasn't in the 1930s-60s.
Sure the AR-15 is a solid rifle, but are you telling me it can't be topped by a completely new design?