The standard way to do this was to sell a license for a specific major version. It goes through a lifecycle where it's initially the most current, then gets superseded by a new major version and maybe gets a few more bug fixes, and at some point one of those releases is the last one for that major version and it's done. Buyers can continue using it forever, but there's no maintenance/enhancement cost, and support can be stopped too if you want.

This worked for decades and continues to work today for some companies. It's not a Ponzi scheme in any sense.