In regards to Office, the business model used to be:

Enough people upgrade Office for no reason other than that's it's there or acquire a new version of Office with a new computer that, because the new Office files are not compatible with old versions of Office, everyone who wants to communicate with those arbitrary upgraders also has to upgrade.

However, the lack of downward compatibility was fixed in Office XP, IIRC. I don't know if that trend continues with Office 2007.

However, the basic issue still applies. Enough people upgrade or get the new version bundled that the new features become requirements through various third-party mechanisms that everyone is forced to upgrade. That applies to both Office and the OS.