A well-constructed subscription system should improve access to new Windows features, something that should both keep end users more secure and make it easier for application developers to take advantage of the latest platform features. It also makes some of the more awkward aspects of Software Assurance better. With SA, Microsoft is pressured to produce a new major version every three years. That's because SA entitles customers to major version upgrades as part of the subscription, with subscriptions typically having a three-year term. If Microsoft doesn't have a suitable upgrade within that term, then the value proposition of SA is greatly harmed. Corporate customers may decide they're better off with perpetual licenses.
The inevitable arrival of subscription-based Windows | Ars Technica