I grew up on iron sights, literally, as a kid, and militarily. There were no "optics" militarily until the early to mid 90's. A few units had the early, EARLY Aimpoints prior to that. My first issued Aimpoint "kit" included a cantilever mount, attached to the carry handle on the M4A1. Nothing before that gave you a co-witness through the irons... It looked like this:
We used them, zero'd them, and trained with them - but you couldn't qualify with them. A few years later, that changed, and you could qual your Aimpoint. By that time, we also had EOTech holo sights, as well.
I have two ARs that don't have back up irons. Two. One is Kim's coyote gun, and the other I haven't had the time or energy to add a set of 45-degree back-ups to. Her coyote gun with get the 45-degree sights, when I get around to it.
My methodology goes for magnified optics and red dots. Doesn't matter. I'm now a firm believer in 45-degree sights for magnified optics, and standard co-witness for red dots.
2 is 1, and 1 is none. If you don't have a back up, you won't even know that you needed it until it's too damn late...