Yes, the term “lifetime” is relative. That said, an airplane regularly hangared and cared for might easily wear the same cover for forty years or more.
About the only thing that destroys fabric is UV and wreckage. Modern fabric systems use a “primer type” coating that blocks UV. Don’t skimp on that stuff and take care of it and there’s no reason it can’t last that long.
And yes, carefully inspecting the tubing is tough with fabric in place, but proper corrosion proofing during rebuild and good care of the plane will go a long ways toward a long life span of the frame as well.
On the other hand, as Denny pointed out, treat the plane like it’s junk, assume you know all there is to flying, and ANY airplane is going to have a short “lifetime”.
MTV