@John *********
The fact that the reason of an extension can change, doesn't mean that the visa is no longer the basis of all extensions. Without the visa there would have been no extension in the first place.
The visa itself is based on retirement, marriage or other reason (the system is flexible, therefore the reasons can change over time after the first year), the extension is in turn based on the visa. In your reasoning, the reasons for the extension are up in the air without a legal basis, which is the visa.