"And even when an old premium car matches new run-of-the-mill vehicle in terms of safety technology, both passive and active safety features detiorate over time: body looses it's rigidy, original air bags unless replaced which is costly are much less likely to work properly after 8 years or so."

I disagree with this strongly. The chassis stiffness and integrity in a BMW is engineered in - try opening the window on a BMW and 'squash' the window opening. It won't budge. Try doing the same thing on a japanese or korean car - it will easily bend to your arm. Same goes for suspension systems. Many BMWs run a full-subframe aluminium carrier for the suspension systems. These are lightweight, strong and isolate road bumps and NVH from the main body. They also last a very long time, unless abused or crash damaged. None of these features are in cheaper cars even today, even if the cheaper ones are stuffed full of airbags.

As for airbags - they run self-test systems each and every time the car starts up, so you can tell if they don't work. A friend of mine recently crashed a 15 year old Honda - and the airbag worked just fine. These are seriously engineered active safety systems designed to work decades after being installed - they're not like the battery in a Nintendo. Every single electronic feature in my 14 year old BMW still works like new, from the sat-nav to the mood lighting, the auto-dim mirrors to the electric memory seats.

Also take body integrity - BMWs and mercedes generally use a double-sealing door seal, and a closing latch strong enough to pick up the whole car. Again, not repeating on new, cheaper models. They're also hot-dip galvanised when built at the factory, so rust is never a problem unless you have poor treatment or crash damage.

I could go on all day. If you've spent serious time underneath these vehicles, the difference in engineering is night and day with built-to-a-price cheap cars. Also, take a wander around a salvage yard where some crashed vehicles are. That will salve your doubts.