In terms of gameplay mechanics, old school zoom aiming achieves the same thing as ADS. Is getting to press your nose against virtual firearms really that important in a game about shooting werevolves and vampires?
I blame this on the proliferation of Call of Duty over traditional scifi and fantasy shooter franchises. You can fucking ADS on an assault rifle in Halo now 
genuinely this, the advent of universal ADS in the FPS genre has made games slower, movement to get closer to your target has been disincentivized, replaced with aiming down long corridors with zoomed in FOV. I think it mostly became this way because of the popular surge of console gaming, holding LT to ADS essentially becomes a "AUTOAIM, ONLINE!" button, and it standardizes switching between two aiming speeds with non-ADS being fast, and ADS being slow and accurate, whereas on a mouse you can instantly switch between fast and slow aim speeds a lot easier by changing how much you move your arm
i think they have their place in a few niche games like rainbow six to incentivize moving slower in combat encounters to make the defenses you place down more of a mechanical benefit, but even in that game in like a month tops people were just running into bases and quickscoping everyone to get around the ADS movement speed drawbacks so like lol, lmao
plus i liked when the gimmick of sniper rifle weapons in games were that they were the only guns that could zoom your screen in, it made them feel unique and powerful
Yeah I think sights work best in games where you're engaging targets at a longer range, say like ARMA, where you can also adjust them to based on range (I think or maybe a mod did that). Corridor shooters or games with a focus on CQB feel snappier when it's firing from the hip.
If they want iron sight realism they need to go all the way and make diopter sights actually usable, i.e. the ring goes out of focus and you get a nice aperture on the front post, instead of how it usually works which is a giant black disc blocks 90% of your screen and you get a tiny dot in the middle to aim through.
Same with notch and post sights, no, it's not just a black rectangle that blocks half your field of view.
What? The distinction between something being plodding and careful or twitchy and fast paced is the available movement tech and how janky a game's hitboxes get when people spam crouch and wiggle back and forth to make them harder to laser down. ADS or its absence is just a style and feel thing at that point.
Like games like Apex and D2 are twitchy and movement focused despite having ADS and generous controller autoaim.
having a mechanic that lowers your FOV and slows the player character down makes for a slower, less movement focused game
Apex Legends is a fast-paced game, but it would be faster, more movement focused game without an ADS system that slows your character down and reduces your FOV
Destiny 2 is a fast-paced game that has weapon perks that exist just so you can move faster while ADSing, but it would be a faster, more movement focused game if it didn't have the ADS mobility/FOV drawbacks to begin with
Apex is slow as molasses. TItanfall 2 and Tribes 3 are where you need to be if you gotta go fast. Destiny is basically a 1968 Oldsmobile Land Cow Deluxe trying to cough and choke it's way up a thirty degree grade in the andes by comparison.
Give Splitgate a try. I believe it's free on Steam. Old school Halo style run and shoot gameplay + zany portal antics like shooting a portal behind someone across the map then jumping out of nowhere to bean them with a bat.
Solid halo shooter + a really fun portal mechanic that gives you a lot of tranversal power and lets you do all kinds of very silly things like shoot someone behind you from behind them, or juke around someone's missile and cause it to go through a portal and hit them in the back.