That's a tough question.
Disregarding motorcycles, for four-wheeled vehicles - and especially for 18 wheeled vehicles - I would say that ABS is a safety-critical system. More importantly, the safety benefit arising from ABS doesn't extend only to the operator & passengers in the vehicle with the ABS system, it extends to other vehicles and pedestrians on the road who don't get hit by a vehicle that has gone out of control due to wheels locking up under hard braking.
Things get more troublesome & hard to justify - this is the "slippery slope" problem - when we look at other safety systems that have recently been mandated for new-build vehicles but have less of an impact on safety of others. I'm thinking of back-up cameras, TPMS, passenger airbag not active warning lights, stuff like that.
If we look at the progression of vehicle safety systems over the past 60 years (since Ralph Nader wrote his famous books), it's clear that the earliest innovations made the biggest difference - things like seat belts, disc brakes, 4 way flashers, front seat airbags, etc. Innovation continues, but the gains from new innovations become smaller & smaller as time goes on. It's a bit like software development... it was a big thing when Microsoft Word introduced a spelling checker with version 3.0, it's somewhat less exciting that last week's online update for Office 365 version 33.6 introduced punctuation checking for 4 dead languages that no-one has spoken in the past 100 years.
Overall, I think that maintaining functionality of vehicle safety systems, whether they are big ones like ABS or less significant ones like back-up cameras, functions as a proxy for how well a vehicle itself is maintained. Back when I was a teen, I could go to Farmer Jones used cars and pick up a schittebox for $25 to use for the next couple of weeks - when the gas tank got empty and the ashtray got full, one took the licence plates off and left it in a grocery store parking lot. No safety checks, no standards at all - if you could start it and put it in gear, it was roadworthy. Those days are now gone, but at the same time, mortality arising from traffic accidents is way, way down.
Michael
I think there is a fairly large gulf, actually an ocean of difference between a commercially operated 40ton transport vehicle and a privately owned 125cc motorcycle. However, legislatively and from an inspection standpoint, at least in my State, if I apply the rules for cars to bikes, they would be treated the same. If that isn't insanity, I'm not sure what is.
The law of diminishing returns is in full force here. Every new safety system added increases the cost of manufacturing, purchase, maintenance, and ultimately even the disposal of the vehicle.
Regarding the "proxy on how well a vehicle is maintained", I'm gonna call BS on that. We'll have to agree to disagree. Beyond my truck, and the known instance of a TPMS failure sending vehicles to the scrap heap, I have another for you. I had a 2006 Toyota Sienna Minivan, with 80K on the ODO. The airbag light came on. Again, it wasn't going to pass inspection like that. I considered the airbags very important. I was quoted $4400 to replace the airbags and wiring harness. At the time about 40% of the value of the vehicle. Who would spend that money? More importantly, why wasn't the system robust enough to last the life of the vehicle? Turns out I found a very easy and cheap fix that maintained full functionality of the vehicle. But I sold it less than a year later. Wasn't going to tempt fate twice.
Most of the crap that is failing now is electrical or electronic. Cheaply made, vehicle/model specific, and the first thing that is no longer available for most vehicles. This has little to do with safety. I'm not sure if a inoperative backup camera will be grounds for inspection failure in the future, but you can be sure they will be expensive to fix if available, and cheaply made by the lowest bidder. If these things are going to be "essential safety systems" then they should be robust, very robust. Yes, it will cost us all a lot more to buy a vehicle, but what price is your SAFETY worth?
BTW, Ralph Nader was entirely incorrect about the Corvair. Gov't investigation into the matter, and subsequent testing showed that the Corvair was no better or worse in its handling compared to its contemporaries. The testing was extensive, and completed in 1971, so too late to save the Corvair. Mr. Nader's entire premise was built on the Corvair being "Unsafe At Any Speed", which was and is a bald-faced lie. I think Mr. Nader deserved a solid punch in the mouth, but hey, that's me.
RT