How can a one-time payment fund a sync service forever?

Because the service is deliberately cheap to run. The app is local-first and notes are small encrypted blobs, so the server does little more than store them and relay them between your devices. No analytics pipeline, no AI features burning compute, no support department.

The one cost that does grow over time is storage, and that is priced accordingly: storage beyond the included 500 MB is a small yearly add-on. One-time costs are priced once, recurring costs recur. The model only has to pay for itself, and it does.