Your example with the brakes is about safety, i.e. making sure the car won't kill you during normal operation. Normally, unless their CPUs start bursting into flames, this is not a problem for Intel.
The problem here is about security. A car analogy would be that to start your car, you need a code and that code can be found by measuring how long it takes to process the input, making life easier for thieves.
As for liability, I don't think you can be liable in court if you didn't plan for something that wasn't known at the time and isn't trivial.
Engineering brakes that work reliably over months and years of use isn't trivial.
Process isolation and kernel security issues have been known for decades and have been fundamental design requirements for decades.
In both of those cases the products can cause serious harm without any third party implication. The brakes would just go of or the battery would explode during normal function.
However, in the Intel case there has to be an attacker that actively exploits an issue in design.
To me this would be like making a class action suit against all lock vendors because they can be bypassed with the right set of tools. The fact that this affects everyone (Intel more than others) and that it took 10 years to find grants them some excuse. Also the architecture is not secret as far as I know so anybody could have audited this. They most probably did do so and found nothing until now.
Now, I do not like Intel communication around this and if it comes out that they knew this for years and decided to sit on it then it would be a different story.
Class action lawsuits are useful when there is negligence, or bad intent but in this case what could it possibly solve?
Sorry, but in the 21st century world of the internet, cracking needs to be taken as a given. In many cases, "normal use" for a computing product means exposing it to use and therefore potential attack from anywhere on the internet. CPUs certainly fall into this category.
Intel is 100% liable to face a lawsuit over this. Consider if a major car brake manufacturer discovered that there was a design flaw in the brakes that prevented them from functioning in certain situations. It'd be facing multiple lawsuits by now whereas Intel is going with "our chips are the most secure ever" line.