Why are older airplanes still in use?
Besides the main points you mentioned, I can think of a couple things, the first is sort of related to your point #1.

Airplanes are designed to be easy to service. Cars are less so.
Cars only have a few very expensive parts, mainly the drivetrain (engine and transmission). If either of those needs to be replaced in an older car, it can easily cost more than the car is worth.

Airplanes are much more complex, and have a larger number of expensive components. The engines of most airliners are designed to be removed relatively quickly. Although the engines tend to be the most expensive part, replacing an engine is nowhere near the total cost of the airplane, and there is generally a good pool of engines out there for parts and replacement (just look at how many JT8D engines are still in service).

In GA aircraft, it is easier for a repair to cost more than the airplane is worth. However, the more comprehensive maintenance required on aircraft means that this kind of repair is less likely to be needed. Also, while its usually easy to go out and replace a broken car with a comparable new or used one, the aircraft market isn't as large. New aircraft tend to be much more expensive, and the used market is more limited.

Another reason is that airplanes can have more unique qualities than cars. For example, there are original 737s flying in Canada because they are able to operate from gravel runways. There are just more combinations of sizes and capabilities in airplanes than with cars. Many times, it is only economical to replace those aircraft (both for the builder and the buyer) in very long cycles.