Spire recently announced that the software is available as a subscription offering on a monthly or annual basis. The perpetual license is still available, with optional annual maintenance to be able to get updates. The subscription pricing includes access to updates.
Subscription offers companies a way to move to Spire without a large upfront payment for licenses. The downside is if you stop paying the software no longer functions. With perpetual licensing, you could choose to coast for a period of time, until underlying changes in Windows or security requirements make getting current a necessity.
Since Spire is actively developed with new features added frequently, there is quite likely value in keeping current, and the subscription option makes it easier on cash flow to get started.