Some programs (e.g., AA) have a "soft landing" policy whereby you're only dropped one level per year. If you earn Executive Platinum one year, you'll be Platinum the next year and Gold the year after that, even if you never set foot on a plane. This policy is long-standing but unpublished, which means it can change at any time without notice or appeal.

Also, some programs have (or have had in the past; Canadian comes to mind) lower requirements to renew/retain a level than were needed to earn it the first time.

If you have a specific airline or a small number of specific airlines in mind, check their forums and/or Web sites. In general, though, the rule is as stated above: fly or lose it. Anyone who is close to having lifetime status in a program would normally be familiar enough with that program to know it.