Combat Evolved came out 23 years ago. That’s older than Super Mario Bros. was when the Wii launched.
Retro isn’t a number. It’s two disconnects. There is always something new - innately distinct, previously implausible, promising of future trends. When new things change enough, stuff that existed beforehand becomes old - tangibly dated, behind the times, automatically uncool. When that new stuff in turn becomes old, the old-old stuff becomes retro - distinct from merely out-of-fashion, illustrative of shifting perspectives, capable of being judged on its own merits.
This is why it’s possible to make brand-new games that are still “retro games.” The indicators of a particular era no longer feel poor-quality or unpleasantly limited once they’ve lost direct comparison to modern novelty. Low resolution is a style choice now that it’s plainly not performance-related. Limited color is an affectation. 3D can be taken for granted, so games doing it badly are doing it on purpose.
I say all this to argue: Halo’s not retro because it’s not even retro. It’s just fucking old. The last big disconnect was in the goddamn 360 era. GTA IV looks like an upscaled PS2 game and GTA V still feels like a mid-budget PS5 game. PBR shading, local lights-- I don’t think GTA V specifically had screen-space reflections, but it was definitely A Thing by the end of the PS3 / 360 era. Volumetric fog was in PS3 launch titles.
Meanwhile so many open worlds have become “one of those games where you fuck a map,” which solidified in Assassin’s Creed. What are the exceptions? Soulsbornes? Yeah guess where those took off. Halo’s just one generation prior to linear titles like MW2. No other MW2. It’s the slightly-lower-contrast, slightly-less-scripted predecessor of a whole bunch of games that were trying to one-up it. Calling any of those “Halo killers” fundamentally distinct feels like arguing “Doom clones” were in a different genre.
Christ, even the retro-as-a-style thing has its inflection point in the 360 era. Cave Story was a big fucking deal. XBLA gave small indie games a taste of revenue. GBA homebrew shifted neatly to shoving emulators on PSP.
It is increasingly difficult to make any game that was unprecedented ten years prior. The toolkit gets wider and wider, but even a sudden massive increase in rendering power wouldn’t allow much that we haven’t expertly faked. I feel like the PS4 came and went without any distinguishing features whatsoever. (I don’t even remember if it was the bold black rectangle or the italic black rectangle.) Contrast this with how Super Mario Bros launched against an Atari that boasted several sprites, and then the NES’s last official game was on shelves beside Tekken 2.
The counterargument to this might be that anything without live-service gacha bullshit is now old. In which case… burn it all down and start over.