Many economical models (including working 9 to 5, btw) came from the times of physical labor. At that time the part of labor was relatively small in the cost st

Many economical models (including working 9 to 5, btw) came from the times of physical labor. At that time the part of labor was relatively small in the cost structure, and the part of materials was big. And since materials is a one-time expense, it was natural to have one-time pricing. In the knowledge-based industry such as software the cost of labor has the biggest share in cost structure. And unlike materials, the labor itself is a subscription. Social policies that make firing hard make it even more so. No wonder that revenue structure also tends to shift towards subscription-based models.