When I acquire a new (to me) bike, I almost always find the mirrors on it annoying. When the bars are lowered from stock, the stock mirrors are too low and far from a casual viewing glance. The stock replacement mirrors are usually flat glass and give a smaller rear view. These are ok, but then everyone appears to be tailgating me, which makes me nervous and often annoyed. True stock mirrors are slightly convex, which gives me a shoulder reference while also what is behind me and in either lane left or right. Plus tailgaters have to be right on my fender to elicit a big worry.
Bar end mirrors require a radical vision departure from where you are going, as they are usually well outside standard peripheral vision. I don't think looking away from forward traffic is being very alert or defensive. And, I like using peripheral vision "triggers" to note changes in traffic status behind me. It's been more than a few times the movement in the mirror alerted me to an overtaking car or motorcycle, preventing me from moving to the opposite side within the lane. Plus, bar ends they make the bike wider, and more difficult and restrictive to lane split, fit through alleys, etc. The only time I ever nicked a car mirror was with a bar end, type.
Yes, I do habitually look behind on either side before whole lane changes. But, those glances are very brief when traffic changes are developing in front of you in spaces that you are moving into.
I will admit, my head swivel motion extremes have diminished as I've gotten older.
