The debate should solely focus on whether reaching a wider audience with a T rating would remove any features, cut scenes, or anything else that could bring value to the game and its ability to tell its story. For instance I dont think the amount of blood or explicit dialogue would have been possible with a T rating for Halo CE which would lead to johnson loosing a good bit of his grit and maybe no longer being a memorable character, that was just the first example that came to mind. If the game as they want it has a T rating then that works but if they have to cut something fundamental to get it to fit into those T parameters then they should stick with M and relying on parents not knowing the difference.
If Halo has to cut content to hit a T rating, they are going to affect the story, probably negatively since it would restrict 343 from telling the story they wanted to. 343 should go for a story that has gritty, moving moments, but also those moments like those with Johnson in Halo 2. If H2 was T, Johnson would have been a lot more boring. Plus, with the flood back, one of the best parts about fighting them was when they broke apart. Can't do that in a T game. Also in lore, the flood are much scarier, and I think they should be somewhat dreaded in Halo Infinite, not because they are bullet sponges, but a kind of fear of them. With a T rating, you can't really get that type of immersion. 343 needs to focus on the game first, and worry about the rating later.
Also, parents should be able to review a game and judge what their kid should or should not play, so it is their choice. 343 shouldn't really worry about that.