It's not irrelevant period, but you are right that it may be irrelevant to some younger gamers who are getting started. They can, of course, become aware of these things through interaction with older gamers though, and may be interested to learn about the history of some of the games that they are playing .
I'm far from what you would describe as a young gamer, and Advanced Space Crusade is only known to me because I've heard it being talked about. I have no idea how it plays or what the rules are. The game was released in 1990, so we need to remember that was 26 years ago. Longer than a good portion of the gaming community has been alive, and longer than a larger portion has had an interest in gaming.
GW is going down the right path by bringing back part of what made their company great. They were more than just "the Warhammer / LOTR company," they had board games in a wider selection of games to play. They are going back to that, and if they have a history of games to choose from that they can introduce a new generation of gamers to, why go out of your way to design something completely new (which they have done with Execution Force, Betrayal at Calth, Renegade Knight, Deathwatch, etc)?