The updating of the OS is on the manufacturer, a manufacturer who's purpose is to sell handsets, not support existing ones for free.
Each update is a carefully calculated balance between maintaining customer satisfaction and shooting itself in the foot by giving customers too much--because phones aren't about the hardware anymore, it's all about the software... software that several generations of phone could theoretically operate (as demonstrated by a 2+ year old iPhone 3GS running iOS 5).
Don't count on 4.0. The Nexus Prime definitely won't, but the Desire S is a bit newer. Plan on a long and painful wait.