That is fast becoming not true.With all other MF systems you are able to buy a new back one day and keep at least the body.
This means buying a S2 is expensive, but updating a S2 to the S3 will be even more expensive-compared to other MF-solutions.
It was when multi-adaptable backs were the rule, but no longer true with any Hasselblad H platform since the backs and body are integrated and have to stay together. With Phase One the new Mamiya based bodies with improved AF are much desired ... although you don't have to upgrade your back to get the new body.
Fact is, they all drop in value like a stone as technology presses forward. So far, history has shown us that the more expensive it is, the bigger hit it takes ... the difference in price gap between the lower end backs and the ultimate backs of the same era closes considerably as new stuff hits the market.
No need to project the future IMO. The S2 is just very expensive NOW ... period.
-Marc