Scopes don't "gather" light. They always transmit less to the ocular than passes into them. I've been shooting long enought to know this one predates the internet, and it's gunstore bs.
Red dots are nice on carbines and close range work, but what someone has failed to notice, they often have 2MOA dots, or larger. That makes them somewhat harder to shoot accurately at long range. The makers don't advertise either carbines or red dots for that anyway. What is used in the military is a rifle with magnified optic.
Iron sights are dying out in public sales because marketers are taking advantage of scopes and red dots as the answer to poor shooting skills. Yes, red dots do work well when the shooter can't consistently get the same cheek weld and hold a sight picture with the post centered in the notch. Yes, if you can see the dot, put it on target, and pull the trigger, the round will hit. Yes, they are much faster in acquiring a target.
NO, they don't do it for cheap, and those who do buy cheap don't get durability - which is inherent in iron sight design. They are made of steel, have protective wings, use robust mounts, and put up with a lot of abuse when made to military grade.
This points out the use of the proper pins to hold the front sight on AR's. Milspec is tapered pins. Those provide an very abuse resistant mounting that prevents the sight - and it's critical part, the gas tube - from moving. Banged on vehicles and doorways, dropped on concrete from shoulder level, torqued and twisted in bayonet training, tapered roll pins put up with the abuse and keep the weapon functioning.
Loctite and set screws need NOT apply. I agree that if it takes loctite on screws to hold any sight on a weapon, it's not designed correctly. That's a tradeoff of the picitinny rail system, and there are levers offered to prevent loosening. Done right, those wouldn't be there.
In this regard, optics are creating a less secure rail mounting system instead of a more rugged setup for iron sights. It's not uncommon for carbine course instructor's to have to recommend a student to strip all the unnecessary stuff off so that it will not litter the range.
Hard advice to follow if you are there to show off what your credit card can carry.