1. These offers are not for your lifetime, but for the lifetime of the company, which may not be that long.
2. If the company does live a long time and becomes successful, it often reneges on these lifetime offers.
Honestly been pleasantly surprised now that they've had a subscription model for years and have honored my app purchase from probably over ten years ago
It's not a technical problem and it's not a contractual problem. It's a trust problem. Your business hasn't established a level of trust with your customers for them to believe you.
Until you have that trust, don't offer a "lifetime" anything.