imagine how CoD encounters play out
you see someone, you stop or strafe and shoot them, then you sprint back into action. that's how ads typically works because it forces you to stop and pop someone. this works well in cod when encounters last a second at most ideally. halo is not that game, but we don't know how ADS functions in this game yet. but for sure, if it functions similarly to other games it will work to the detriment of the Halo formula. Halo isn't about stopping to aim at someone, Halo is about keeping in motion and using movement intelligently.
Halo is more about tense little skirmishes involving movement, zoning, using level geometry as cover, etc. while trying to control the map and power weapons; weapons use the 'camera zoom view' to promote hip aiming because it's hard to strafe and avoid fire, grenades, or flanking while you're zoomed in. its a balance of these things that gave halo its distinctive feel where encounters are pretty fair and skill centric and knowing the map and having your own skillset was basically a surefire route to victory. Even if you're shot first you can take a fight back with some cleverness. ADS could potentially change this balance and remove some of the emphasis on intelligent motion, homogenizing Halo with this shooter design that's been eating up every other major AAA series. Sprint has an effect on encounters too, with sprinting (among other features) zoning is pretty severely impacted, and level design has to compensate for bursts of sprint, leading to longer hallways and gaps between cover, and larger less dense maps in general.
I could deal with just sprint but unless ADS is pretty much useless (no more accurate than hip fire, doesn't impact movement, just a visual alternative) it won't be doing any favors to Halo's design.