multi-role aircraft versus multi-variant aircraft

There’s a lot of talk about how complicated multi-role aircraft have become, expensive avionics, long and delayed development process, and the ability of the pilot to focus on multiple roles and become a jack of all trades master of none instead of a specialist.

you got your fraction who want dedicated aircraft and dedicated pilots but that is also expensive..

what about the attempt to use multi-variant aircraft of the same type.

i.e. JA, AJ, SH, SF Viggens and Tornado ADV/IDF?

instead of: F/A-18, Rafale, Eurofighter, Gripen
or
Su-25, MiG-29, MiG-31