When games like Duke Nukem 3D or Quake were out, Boomers were what? 30 to 50 years old?

You mean, like most of us Millennial gamers are now (30+)? The youngest Millennials, born in 1996, will be 30 in 2 years.

These games clearly took inspiration from 90s FPS games, which 👌, but they were played mostly by Gen Xers and Millenials, not Boomers.

I'm a middle-Millennial (1988), and Doom was well before my time as a gamer. I was 5 years old in 1993. Halo (2001) was more my generation, just barely. The oldest Millennials in 1993 were 12 years old, which was not the target age group for Doom.

GenX? Sure, they played Doom, but Boomers were by far the larger age group playing "Mature" games at that time. Video games have never been just for children.