Some people aren't bothered by a fixed rear while using the optic. I tried it that way at first but had to go to flip down. You just get a better field of view. Also when you look through the peephole the red dot dims. That's easy to see with bottom third cowitness.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek