Upgrade vs New

Ok lads , iknow this has probably been posted before but…

Are there more benefits to upgrading older fighters over purchasing more modern types?I will illustrate this by choosing two fighters , the Cheetah and Tigre F-5 upgrades.Argubly the cheetah being more impressive.In addition to a modern Radar by Elta , it has a comprehensive new avionics and EW suite , inflight refuelling probe , additional weapon pylons, and a host of aerodynamic mods such as canards , strakes , clearvision canopy and multifunction displays and HOTAS.I know the Tigre has a new radar , avionics , multifunc displays and IFR.

Now I know that none of their powerplants have been changed apart from making the Cheetahs Atar more reliable and easy to maintain.I also know that although the Cheetah was comprehensivly rebuilt with zero hour fatigue (50% airframe?)that new built has advantages of long airframe life.Argubly though , flight performance parameters have hardly changed since the 60’s , service ceiling , speed , climb rates etc. SOOO…Equipped with all this new gadgetry and modern AMRAAM type missiles and cruise missiles in the Cheetahs case , there seems to be a strong case in radical upgrades.NOTE: I’m not comparing these to the Rafale , Euro , F22 etc , but rather to F-16 , LCA , Mirage 2000 etcI’m aware of FBW etc , but surely the strike roles and intercept roles can be carried out as efficiently , and dare I say , in some instances , slightly better!!!Hee hee..please guys , lucid and intelligently to the upove posed questions..