The only reason this government mandate came about is because of other government mandates, namely safety mandates on newer vehicles that eliminate rear visibility. I drive 80s trucks and 60s cars. Excellent visibility all around. Aside from a few fastback body styles which limit blind spot visibility, but even that is much better than most new cars I've been in.
New cars have very high door panels, and thick/wide A/B/C pillars making windows much smaller. There are also front seat head restraints, and in the past few years rear seat head restraints as well. Good luck seeing anything out the tiny windows past that maze of DOT/government mandated view blocking devices. Now check out those tiny side view mirrors they use these days, virtually useless. To make matters worse the glass is curved to magnify the image, give a narrower field of view in an already tiny mirror! I feel claustrophobic and blind in the rare event I drive my girlfriend's fairly new car. Believe it or not she doesn't bother turning her head when changing lanes, and I kind of understand why... You can't see a damn thing looking over your shoulder anyhow. None of my fastbacks were ever that bad and they didn't even have mirrors on the passenger side, and not once did it ever occur to me to desire one on that side as it simply wasn't necessary in a vehicle you can see out of.
The problem is government induced. Government mandates safety "features" that people don't want (if they were cars would be offered with those features and sell well), those safety features result in limited visibility in all directions. With limited visibility in all directions, especially behind, pedestrian strikes increase. Government mandates more things people don't necessarily want.
This reminds me of the government interference in the 70s. Government mandates safety features, which tremendously increase the weight of cars reducing MPG. Then they mandate emissions requirements, which greatly reduced MPG. Then they mandate MPG requirements... etc... In 1960 economy cars were getting 32+ MPG and selling well. What was the problem? People had a choice of whether to buy the small car that gets good MPG, looks nice, has decent power, and so forth, or big a bigger less efficient car which had great power, looked good, etc. The problem was choice, so government outlawed choice and the free market and the result was small cars that got low MPG and were hideous.