Realism doesn’t have a time stamp!
As a champion of ultra-realistic flying, I do see your point about liveries that never/have not yet existed (Delta’s B788, Emirates & United’s A350s) but getting rid of old aircraft just doesn’t make sense.
Clearly, it’s not just about keeping up with in-real-life realism, but also about accepting what once was and respecting the history that has got us here.
In my opinion, getting rid of old aircraft or not adding retired aircraft simply because they are no long used is short-sighted and ultimately egotistical towards modern aviation, one of the many problems with contemporary aviation enthusiasts.
Just my 2 cents though!