This is a very interesting problem - how to be fair to the users in case recurring business costs are zero or negligible. Can you not support the newest versio

This is a very interesting problem - how to be fair to the users in case recurring business costs are zero or negligible. Can you not support the newest version only? If anyone needs help then they would need to update. Some people could keep-up with subscriptions, other would stay with the outdated software that was good enough for them. This would mean customers pay larger price once (for a "license" to own app X indefinitely) and then a smaller update fee every month/year (subscription) that is completely voluntary. Do you have some custom file types or internal data that could have changed between versions and would be hard to migrate if someone decided later they needed support after all?