I think Russ has explained the complexity issue well.
Heavy trucks and large buses generally have full air suspension systems, but it is worthwhile for them, they keep complexity down with fixed-height control valves, and they have an air compressor anyway for the brakes.
Originally Posted by ruscal
A friend of mine has a BMW x5 with air ride height control. It was problem prone. He was quoted $2500.00 to replace the right front air bag strut from BMW. He got tired of the problems and removed the system completely.
Kits to replace air springs with coil springs are readily available for some vehicles - while most work fine, clearly a significant number of people have issues.
On the other hand, BMW considers buyers of their vehicles to be captive and deep-pocketed sources of parts and service income... I'm sure the dealership was looking forward to this job and would be disappointed if they never failed.