Tell the truth - when was the last time you actually used your BUIS?
I know when last time I used mine was...when I installed them and zeroed them.
Since then I've needed them exactly zero times. I replace the battery in my Primary Arms Holosun-made reddot once every 8-weeks and leave it set on 3 or 4 out of 10. I've never had the dot actually die. But I turn it off at the range and shoot targets through the tube every time, just to make sure I can do it.
Here's an unpopular opinion - If you're running a hard-mounted optic you don't need BUIS.
Note the context DEFENSIVE AR15. If you're running a duty gun which may be employed for offensive purposes, you should probably have redundant sights, but only if you have a QD-mounted optic.