I don't know about the US, but in general there are strong financial and marketing incentives to upgrade to a new car, in the form of government subsidies (to keep the market alive, despite sometimes masquerading as green new deal) and marketing (advertisement, social status considerations).
Manufacturers make it almost impossible to order parts for older vehicles, and make sure they keep releasing new models whose parts are slightly incompatible with one another (like smartphone manufacturers do).
Cars sold in the last 10 or so years make it impossible to change parts without a manufacturer-approved (or a foreign-language pirate) debugger firmware.
Also, modern cars are equipped with tons of electronics who each have their own ways of failing over time. Dozens/hundreds of different chips and processors serving questionable purpose makes your product overall less durable. The era of "my cousin will fix my car given the right part i can order from the store" is long gone.
Maybe the original parts from a brand new car are overall more reliable than in the past (i'm sure they're more energy-efficient, but i'm not convinced they're more durable) but the car as a whole is much less durable and the car industry is a very good example of subtle planned obsolescence.
Planned obsolescence does not only mean designing a specific part to break over time, see also https://en.wikipedia.org/wiki/Planned_obsolescence#Variants
Because new cars are being constantly sold and you said "average age of cars" when you really should have said "average years of total use", which is about 20 years and is growing. Not 10 years.
If this is true, why is the average age of cars in the US only around a decade? If it really is more economical to constantly repair old cars, you'd expect the average car age to be in the decades.
>If it ever does, it sure is a demonstration of planned obsolescence (voluntary bad engineering to push sales).
AFAIK cars have gotten more reliable in the past few decades, not less.