Tomorrow marks the end of the upgrade era.
Subscriptions are good for Microsoft because upgrades don't matter nearly as much as they used to, as witnessed by the lengthening of customer upgrade cycles. The differences between Office XP and whatever you are using now are mostly cosmetic and in improved ease of use. Yes, these changes matter, but it's not like XP makes Office tremendously more useable than it is already. Changes do add up over time. Still, Microsoft can't count on the upgrade revenue it once enjoyed yet still needs to invest to support .Net.