I tend to agree, however, I also disagree.
Every bike I've owned, I've been still able to see a vehicle directly behind me at a traffic light, as well as still view the left and right lanes to the "rear".
The "Street" bike mirrors appear to be too close together prohibiting this "view", as I'll call it. I have great visibility to both my left & right, but, until I changed things around I couldn't see you if you were bumped up against the rear license plate, pushing me. I just wouldn't know you were there. Too "me", that's a safety issue, sorry.
I want to see "all around me", front, right, left AND rear.
My normal ride is my XL-50 Sportster and it's considerably larger than the Street. The mirrors are farther apart but only by 2 1/2 inches and guess what. I can't really see behind me on my Sportster either.
I've been on all sizes of motorcycles and yes, the larger the motorcycle typically the better the rear visability and the Street 500/750 has about the same rear visability typical to that size of motorcycle.
What's directly behind you doesn't really represent a threat and when it does there generally isn't anything you can do about it. If you're stopped with a vehicle in front of you and someone's going to rear end you there's not much you can do about it. Even if there's no one in front of you you're not likely to pull into cross traffic to avoid being hit. Riding down the road you'll become aware of tailgaters that do represent a threat but the only real remedy is to pull over and let them pass and for that you need side mirrors.
Clear vision ahead and being able to see to both sides behind you is imperative in any vehicle for safety reasons but that same reason doesn't really exist for seeing behind you. My 1979 Chevy Van doesn't even have a rear view mirror and only has side mirrors and it'd not dangerous to drive. I can't even see as much behind me in my van as I can on the Street.
Yes, you can put extensions on any motorcycle and it will increase how much you can see behind you. You could even get absurd and put two foot extensions and see virtually everything but it's stupid looking and not really required for safety reasons.
I can understand why some want to see directly behind because you can do that in a car but a motorcycle is not a car.