What is the most common end of life issue for a car?
Does electrical issues cause end of life most frequently? Maybe transmission? Alternatively cracks in the chassis?
Depends on how you consider the value...
An insurance adjustor would just look at value of vehicle vs. damages to repair. So, common thing that causes vehicles to get totalled via an insurance adjustor is expensive repairs like.. air bags deploying. Those things are expensive to replace.
If you're looking at the value of the car from a practical / usefulness stand-point...
It depends on the vehicle (make / model).
I had an old 80's mercedes station wagon that KBB'ed for about $1500. I loved that car, but it was a money pit. Seemed like shit was always going wrong with it, and every repair was $1000. The biggest "end of life" moment for it was the vacuum system going out. It seems the mercedes company was big on using the model I got to do extensive vacuum-related activities... using vacuum to do everything from door locks to cruise control. So, it had extensive vacuum hoses under the dash and such. When you got a vacuum leak.. ugh. Eventually car was so old the leaks just started piling up. I paid over $3000 for Autohaus to redo the hosing, and it STILL got leaks after.
So, if the car has some weird "we're gonna test out this new way of doing things!" concept the manufacturer did, chances are good that's going to be the achilles heel. (because in regards to that mercedes.. the engine was built like a brick house.. but there were just fatal flaws.. like the water pump position in such a place to melt the bottom all the time and cost $500 to replace, and the extensive use of vacuum causing rotting hoses to eventually make the car difficult to deal with.)
There are other cars that just keep chugging as long as you do major overhauls. Toyota's are big in some countries. They just rebuild the engine or transmission every 100k miles, and possibly replace them every 200k-300k miles.
Theoretically, electrical system in a car (ie: wiring harness through-out car) shouldn't go bad as long as it's quality (sealed wiring, installed properly, and no weather gets into it and no bugs / animals chewing on it).
You can have chips go out on cars, but if you can track that down, you just replace the chip.
The way most folks consider "end of life" for a car is when the cost of up-keep out-weighs the cost of just buying a new car.
EG: if you're paying $300 / mo to repair the darn thing to keep it road-worthy ($300 x 12 mo's = $3600/yr) .. you might as well buy a new car and just have a $300 car payment.
But, it depends on what a person can afford.. and how inexpensive it is to repair a car.
While I got stuck with my money pit mercedes, I had a friend driving an old 80's chevy pickup that had >200k miles and just kept going. He spent maybe $100/yr on it for some piddly thing that would go out. He'd fix other stuff himself (eg: the mirror got ripped off one time, so he bought some aluminum bar stock and just bent a new bracket for himself, drilled some holes.. mounted the mirror back on.)
Most people consider engine and tranny to be the big things going wrong to sell off a car.. b/c those tend to be "thousand dollar problems" that folks would rather chuck a thousand dollars towards a down-payment on a new car.
But, electrical can definitely be a problem.
I had an LHS that had an electrical issue where if you stepped on the gas to kick it into highway gear... it would blow a fuse and engine would die! (I learned that the hard way after almost dying on the freeway when it did it the first time.) Thing was a death trap due to that electrical issue.
If there's an issue with the vehicle that makes it unsafe to drive, and is impossible to track down.. then you have to consider the value of human lives are more valuable then the vehicle.
But, the vehicle can have other issues that seem impossibly to fix, but the car keeps going. Sister drove an old car that burned oil like crazy. She'd just put a quart of oil in it every week and kept driving it until she got in an accident and it was totaled.
So, really, "end of life" for a car is subjective, and a lot of time it boils down to how much someone can afford to repair, how often they have to spend it, and that countering how much it would cost to simply buy a new car (either out-right by saving up or getting a monthly car payment).
I'd say being written off in an accident (where the cost of repair is greater than the total value of the car)
2nd to this, its usually engine issues that end up costing more than the value of the car to repair - issues related to the engine block, engine head/gaskets, consuming large amounts of oil etc
Availability and affordability of suitable fuels will slowly push older vehicles off the road.
Consider engines that required lead to lubricate valve seats - without lead or lead replacement fuel, or expensive hardened valve seats, they would erode the metal and eventually have sealing problems. Repairs were possible but were major head work and had a high labour cost.
Similarly, my old landrover has been converted to run on LPG, which runs nicely. However fewer service stations carry it so eventually I might not find it within a half-tank range of home. (yes I know it can run on petrol/gasoline as a range extender.)