I have no idea why people are clinging on to Halo that hard. If you are an older gamer - which I am now - you have seen the old school shooters of your youth all die a slow, painful death. That would be Quake, Unreal Tournament, Painkiller, Alien vs Predator, Halo, WoW PvP. All of these games looked like they would last forever, and they just didn't. Now, none of these games and the philosophy they represent is truly dead, they live on in niche, custom, community run servers. But mainstream? Nope.
And why would they? The Halo nostalgia specifically is tied to a specific, historic social group. The 14 to 25 year olds of the 2000s, who often had their first multiplayer experience with Halo and the XBox. That was almost easy to do, with far less competition than today. These players aged out, got jobs, finished colleagues, got married, got kids and mortgages. That's not an issue, if you have new players coming in. But they didn't come, not with League of Legends, WoW, Call of Duty, Roblox, Fortnite, etc. taking them in. Today, kids would be bored to death by Halo, looking at Tiktok verticals during quieter moments.
In an alternate universe you could have managed the Halo franchise properly by adopting the formula to modern tastes, but that is hard to do. Probably impossible with that much competition. You have brands or franchises who did survive for decades, let's give James Bond, Harry Potter or Call of Duty as examples. But even those are dead or declining as we speak. Adopting the formula only goes so far. If Halo yesterday was a shooter sandbox, then a PvP MMO, then a MOBA, then a battle royal, then an extraction shooter, then a Netflix show, now a PvE team shooter ... you don't have a brand anymore.