I’d argue that it’s the fluidity of movement and gunplay that made RTCW and ET great. Sure, class and objective based play was key too, but it was that combined with the twitching, tracking and movement from Quake that turned these games into the super aggressive and fast team-based fps most people here love. Years later this still sets these games aside and makes them special - they’re still talked about in all of the PC gaming forums I frequent. Which brings me to iron sights, which are the devil.
Iron sights don’t allow for this kind of flow. They break it up and slow the tempo. Just the impact of having to stop or slow to a crawl to shoot is massive - gone is the dance of twitching from target to target and tracking them as they move, and in it’s place something more fragmented and chance based as static targets spam away with bouncing guns taking up half the screen. Iron sights discourage aggressive play (bursting into a room full of people and stopping to shoot not being the best tactic in the world!) and put more emphasis on caution and defense (which some people like to think of as clever play, but what we really know is camping and hoping to see someone before they see you 
). They are one of the two great equalisers in current fps (the other being low health) and are a great method for lowering the skill ceiling (which, in the name of accessibility, is why many modern fps have adopted this way of play). Make it easy to kill and so random that luck plays a large factor and more people will be able to do well, be happy, and pay you more money. To see this refined to a tee you just have to play CoD, which is essentially a Benny Hill show with guns, with lots of people running around, shooting each other in the back, and getting kills with a few hits to the knee. Rinse and repeat, mask any sense of real progression with unlocks, and job done.
So what’s the problem? Just make a game without iron sights. Easy. Ah, but why don’t we have the best of both worlds - we could marry hip firing with iron sights! Wouldn’t that be great? No. It won’t be. And here’s why - it will be a balancing nightmare that impacts how the entire game plays. To keep any sense of the combat enjoyed in RTCW/ET (I’m now getting Splash Damage specific and assuming this is a goal for future projects) hip firing will have to be viable to a fairly decent range, making iron sighting a handicap as people get circle strafed and minced. Suddenly the forums are filled with people crying for smg nerfs. And why not, most have grown up with games like CoD and either don’t know any better or like that kind of lottery play. So, smgs start to get ‘balanced’ and hip firing accuracy/range gets reduced. Now running about with an smg is like having one of those weird pisses where streams go off in all directions. Out goes precision (along with the competitive community) and in comes random luck. And what about player health? - iron sights really aren’t suited to the bullet sponges we had in RTCW/ET, especially if the game has any pace. That will have to be ‘balanced’ too. But now we have iron sights, random firing smgs and people with not much health running about - whoops, we have CoD with classes! And would that be so bad, at least from a monetary point of view? Try to take what was great about old games and cross that with what’s popular in current. Maybe not, but you run a serious risk of producing a game that doesn’t know what it wants to be, pleasing neither old or new players and scrabbling with other companies for scraps from CoD’s table. It won’t be a game that makes a mark, and it won’t be a game that years later is still remembered as being special. Seems like a lot of hassle when you’ve already got a near perfect template to work with 