steel is great except for how easily it rusts. there are regions on the planet where a car shell is rotted out in 10 years. if a shell could be made from titanium you would have a long life vehicle, with environmental and economic savings.
Citation needed. This depends very much on the alloy, but I would expect titanium cars would be forced scrapped after 200,000 miles (most of my cars reached 200k miles before they reached 10 years old) by law because fatigue builds up in normal use and the car is liable to break apart. Aluminum has the same issues and commercial trailers track how much the trailers are used and scrap them.
Steel has the nice property that if you stay under certain stress limits fatigue doesn't built up over time and so you can keep using it as long as you care to (or until salt gets it).
How fast a car will rust depends a lot on the country where it is used an also a lot on whether the owner has a garage where to keep it.
There are many countries where only a small percentage of the car owners also have garages, so the cars stay always outside, in rains and bad weather. Such cars rust completely far quicker than the cars kept in better conditions.
I had a car that I have used for 30 years and many hundred thousand miles, without having a garage. By its end of life, it still had many parts of the original motor, but from the original steel chassis there was nothing left. Every part of it had been replaced several times, due to excessive rust.
There is a lot more than that. Washing a car to get the salt off can make a big difference. Iron can be galvanized to prevent rust. Different alloys rust at different rates. Those are things I know about and I'm not even in the field.
Stainless steel is cheaper than titanium. Even if the price difference between titanium and stainless steel is likely to become smaller, it is most likely that stainless steel will always remain significantly cheaper, especially in the form of alloys where nickel is replaced by manganese and a part of the chromium is replaced by aluminum.
Unfortunately, even stainless steel is considered as too expensive by the car manufacturers, despite the fact that when we consider the total cost over the lifetime of the vehicle, with the need of replacing the rusted parts, the cost of stainless steel could have been less (but then customers would have been repealed by seeing higher upfront costs, without knowing how much they will spend on repairs in the future).
