Times when red dot sights, also known as reflex or holographic sights were big and expensive are in the past. Do you know that they were used on fighter aircraft in limited numbers during World War I and widely used during World War II?
Optic and red dot sights are not that expensive as it was before. They are compact and inexpensive. That is why they are widely used on rifles, shotguns and even handguns nowadays.
But iron sights are still used by Marines, Army and competitive shooters. So why are they used, and what iron sights are preferred for your AR-15 rifle?
Iron sights are much more difficult to break. They can be used during rain, snow and even when wet or dirty. They can take a lot of abuse and still work and enable the shooter to aim and hit a target.