ACOG and iron sights as backup. It’s not always about batteries—sometimes they break.
Hell, you can get an Aimpoint holographic sight that runs on AA batteries. Nothing more common than AAs!
My Burris fixed 4X uses a 2032 type battery to light up the reticle in the dark, but even “off” the reticle is very good. As in, after all the batteries are dead, it’ll still be a good optic.
And nothing more common than AA batteries barfing inside whatever they are left in, destroying the unit.
I have tried to sidestep this well known liability by using lithium AA in my long term small flashlights and GPS, but there goes the "readily available" cachet.