I had 2 peep mirrors on my chopped 49 Buick. The drivers side I kept, took off the passenger side as I could not adjust it to see where I needed to see. It was just bent so It would not adjust to where I needed to look. Since my back window is so small and at a bad angle I installed a backup camera. It only comes on when backing up. I have been thinking about hooking it up so it comes on with the right turn signal, or install another camera that looks down the side of the car.
I added Fatties to my peeps on my avatar and they made a hell of a difference. No blind spots, stay tight and never have to readjust. The Fatties are a bit spendy but well worth it IMO,
my wife's '50 Chevy PU had shaved door handles so, did not want outside mirrors. used wide convex suction cup mirrors attached inside to windshield on both sides. could see if something was at sides of truck. not for towing, ha!
That's the thing, most people don't have their outside mirrors adjusted properly to where they'll do the most good.
I've had a few clients over the years who want their side mount spare mirrors always 90 deg from the rest of the car, facing straight back. One of them said they look stupid adjusted so you could actually use them. Yes, that's what he said. I had to walk away when he wanted me to help him back it into his garage. "You have to wait, I gotta take a leak." He tried, but wait he did. Of course I didn't have to go, but what am I sayin here, you already knew that right?
I pulled a car trailer around for a lot of years, having side mirrors that function are pretty important to me, and I know how to adjust them. For the most part, peep mirrors are junk. Back up cameras are helpful, but having them on all the time, to be used as rear view mirrors, requires a bit of personal adjustment. That constantly changing screen takes a bit of getting used to, and if the guy behind you is close, most of the view from the camera is blocked. Add to that the fact that the screens don't often match up well in a traditional interior, they can be a challenge. Another form or function choice. The camera in my coupe only functions in reverse. The camera in my truck can be turned on or off any time the ignition is on.
If you use convex mirrors, either small ones on the lower outside corners or the same o.d. as your "peep" mirror, they take some getting used to. I've used them on many cars & trucks over the decades with safe results. Just don't make split second decisions when changing lanes.
I have found it is all about learning how to use them. I have been driving a 39 gmc rack body truck since 1991. I haul stuff and pull trailers with 4" round mirrors. anybody who drives it says I need larger mirrors....but I don't! I just learned what I am seeing. I do move my head to "get the whole view"
I heard Alan Greenspan (Federal Reserve Chairman 1987-2006) once say that "The Fed" uses trailing economic factors to steer the US economy, referring to employment, spending and consumer confidence reports. He said it was akin to driving an automobile using only the rear-view mirrors. Always liked Greenspan, I did well under his watch.
I have 3" convex peep mirrors on both sides of both of my cars, and visibility is great. I can easily see what's beside me when my mirrors are adjusted correctly. I adjust mine so the inside edge just sees the edge of the body on the side, and I don't point them to see a mile back, just a block or less. Never had any close calls, or missed seeing a car beside me or approaching.
My passenger peep mirror is as much use as an ashtray on a motorbike but suits the look of the car. I turn my head when I need to. To my mind its all part of of the idiosyncracies of classic motoring as are other things such as no seatbelts, warming up the engine before driving etc etc. To my mind classic motoring and its ergonomic limitations make for better/safer driving. For what its worth, most of the factory mirrors on UK 50s 60s and 70s classics were pretty useless.