I think the best way is you buy a software, and that version is supported "forever".
The developer then creates version n+1. The old version is kept supported, but new features go only into the new version, which you can optionally buy again.
Time/energy wise, even with agentic coding, that's probably not the most fun value proposition for smaller/solo dev teams. I now have to maintain a mental model of several versions of my software, track features, refactors, etc across all the supported versions, and make sure my work doesn't overlap too much lest I cause more bugs while keeping everything stable.
I wouldn't charge customers _less_ for that just because it's now a one-time payment.
You can pay to unlock advanced features and keep them and any new features added in a year, after that any new features are paywalled for another unlock, and another +12 months, perpetually.
The developer then creates version n+1. The old version is kept supported, but new features go only into the new version, which you can optionally buy again.