Surely you can see that the best thing in an ideal world would be that armies are equal, regardless of whether the models were made before or after AoS. Brand new armies are no more deserving, or no more part of the setting than old ones, they just happened to be thought of after the new game was made.
It would be no better for the game for a new player to pick up CoS or Skaven and find that they’re much worse than Nighthaunt or SCE.
In a few years should we make Idoneth Deepkin etc worse so the new stuff then is the best?
Games Workshop isn’t good at balance. Sometimes brand new stuff comes out too strong, sometimes it’s rubbish. It’s indiscriminate.
Advocating for them to be balanced is one thing. Advocating for older armies to specifically be weaker than newer ones seems bitter and vindictive.