In my mind, the issue isn't so much that CoD was this insanely better product. But at it's core, CoD has parallels with sports games, but instead of players, it's guns. Mil-Sim as a category is something that I think Halo lost to, not necessarily CoD if that makes sense. At one point in time it was going to be Battlefield, which is also another mil-sim. And I think the idea of what Halo is, the setting and style began to alienate it from success, not propel it forward.One gripe: Halo didn’t ‘lose forever’ when CoD came on the scene, and Xbox certainly didn’t. CoD4 came out in 2007 alongside Halo 3 and both did extremely well. Halo Reach came out in 2010 and also did very well. The own goals started with 4, but that had nothing to do with CoD, it was Bungie going off and doing their own thing. Similarly, CoD entered into a pretty long period of stagnation shortly after, as most of the Xbone/PS4 era CoDs were not well received (starting with Ghosts and continuing throughout the ‘jet pack era’). They didn’t really come back into vogue until MW2019 at the tail end of the generation. Warzone would come out at this point, and that’s when they went battle royal (although obviously there’s still the traditional multiplayer too). This wasn’t their first BR mode btw, that would be Blackout in BO4, but the fact we all forgot about that is a testimony to how forgettable that era is (with a notable exception in BO3, which many consider to have the best zombies, and is one of the most popular entries on PC to this day partially due to zombies workshop map support).
Nearly all of our biggest GaaS fps titles are mil-sim. There are some exceptions, but none that operates in the 'arena' space like Halo does. I've noticed that every single time CoD or BF move away from modern mil-sim, the response is significantly worse. People love their guns, I guess it's not really all that different than playing racing games where your favourite cars are in it.