MiG-21bis had that in real life. DCS implementation of ASP-PFD-21 is sadly a placeholder.
As for "how could they abandon technologies" - its a good example of doctrine change during the generations of fighter aircraft. The first serial MiG-21F13 of the late 50s early 60s had an internal gun and a computing gunsight. Then it was decided that who would ever need a gun if you have guided missiles, and that nobody will ever merge because constant mach 2. PF, PFM migs didnt have a gun and had the same sight you can see now in DCS Mi-8 just to aim the missiles. Vietnam, Israel and other hotspots proved that wrong. Later 21s(1964 MiG-21S and on) had a gun and a computing gunsight returned. Still, dedicated interceptors like Su-15 still had that simple crosshair, and MiG-25 didnt have any sight or HUD at all.