Not making them mandatory means they remain expensive extras, more expensive than if required on all cars. That means second hand cars with these features will be significantly more expensive than those without. Which means that some can never afford these features, while others have to do without other things to compensate. Either way, quality of life is reduced - and more than in your scenario, because the features do not benefit from economy of scale as much when they're optional.
If they were rare they might be more expensive, but otherwise it's the same system and will cost about the same amount to incorporate into a vehicle. Making it mandatory doesn't make it any cheaper. (Usually the opposite, in fact. From the POV of those manufacturing the safety systems, there's nothing quite like having the government mandate that people buy your product.)
Economy of scale is not guaranteed, or even really all that common once you get past early research and development or one-off custom products and into the realm of mass production. In general commodities become more expensive in response to higher demand, not less. If these systems are something most people actually want and consider worthwhile then they'll already have the benefit of any economy of scale which may apply without the mandate.
Mandatory safety features, on the other hand, are a direct example of a negative externality: Those wanting the safety features to be mandatory get what they want while other people bear the cost.
> That doesn't mean we paralyze ourselves with inaction or always prioritize marginal improvements to safety (even others' safety) over all other concerns.
Frankly I don't care if a (private) road owner wants to require specific safety equipment on the vehicles allowed on their roads. Their property, their rules. Other potential users of the roads can then decide whether they're willing to accept the residual risk. But that argument depends heavily on those who are being excluded (for lack of required safety equipment or low risk tolerance) not being forced to pay for those roads' construction and upkeep. It doesn't work for a government mandating systems to be installed on all vehicles, or for tax-funded roads.
You’re asserting strongly that the changes are marginal. And sure, the efficacy of the mechanism should be considered. But the point remains.
Your individualism is irrelevant.
On the other hand, making them mandatory makes cars more expensive, which means (especially during a shortage) that some can't afford them at all while the rest are forced to do without other things to compensate; either way it reduces their qualify of life. But of course the government knows better than those actually affected by these laws where the ideal balance lies between safety and QoL.